Agriculture, Ecosystems and Environment 137 (2010) 241–250
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Agriculture, Ecosystems and Environment
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ariation in total biological productivity and soil microbial biomass in rainfedgroecosystems: Impact of application of herbicide and soil amendments
ratibha Singh, Nandita Ghoshal ∗
entre of Advanced Study in Botany, Department of Botany, Banaras Hindu University, Varanasi 221005, India
r t i c l e i n f o
rticle history:eceived 11 November 2009eceived in revised form 18 February 2010ccepted 19 February 2010vailable online 17 March 2010
eywords:erbicide
a b s t r a c t
Total biological productivity and soil microbial biomass are important characteristics to describe sus-tainable agroecosystems. We investigated the impact of herbicide, alone or in combination with soilamendments, on crop and weed productivity and microbial biomass in a rice (Oryza sativa)–barley(Hordeum vulgare)–summer fallow rotation in a tropical rainfed agroecosystem. Total net productivity(TNP) of crops was greater with herbicide (Butachlor) + chemical fertilizer (NPK), herbicide + farmyardanimal manure, and herbicide + green manure (Sesbania aculeata) than with herbicide + crop residue(Triticum aestivum straw), herbicide only, and the control without amendment. Weed TNP was highest
in the control, lowest in herbicide only, and intermediate in combined herbicide and amendment treat-ments except herbicide + crop residue. The sum of crop TNP and weed TNP was highest in herbicide + greenmanure and lowest in herbicide only. Microbial biomass C and N were greater with herbicide and amend-ment treatments than with herbicide only. Microbial biomass showed distinct temporal variation. Yearlyplant input to soil had strong correlation with microbial biomass in the control and herbicide only treat-
ystemand m
ments. In these agroecoshelp sustain soil fertility
. Introduction
Long-term sustainability of agricultural systems has nowecome a major issue of global concern. Size of agriculture’s foot-rint has increased tremendously mainly due to its role in globallimate change and degradation of natural resources (Kiers et al.,008). About 80% of the total cropland area of the world is estimatedo be under rainfed farming conditions (FAOSTAT, 2005). Rain-ed agroecosystems have generally low crop productivity due toimitations of soil moisture and nutrient availability, yet immenseossibilities exist for increasing crop production in these extensiveainfed agroecosystems. In India about 66% of total arable land isnder rainfed farming conditions.
Weed infestation is more severe in rainfed than in the irrigatedroplands (Hyvonen and Salonen, 2002) and herbicides are usedlmost exclusively to control weeds. To maintain fertility of these
oils, addition of exogenous soil amendments is generally required.n rainfed agroecosystems, addition of organic residues rather thanhemical fertilizer, holds promise to ameliorate soil fertility as wells enhance soil moisture content. Differences in resource quality
∗ Corresponding author at: Centre of Advanced Study in Botany, Department ofotany, Banaras Hindu University, Varanasi 221005, India. Tel.: +91 542 2392678.
of soil amendments can affect available nutrients in soil (Singhet al., 2007). However, when soil amendments are applied withherbicide, interactions may cause alteration in the effectiveness ofherbicides. This may also result in changes in the factors responsi-ble for growth of both weeds and crops (Jenkinsen, 1988), therebypotentially affecting crop yield.
Maximizing crop yield is the major objective of all agriculturalmanagement strategies. Crop yield, however, constitutes only asmall fraction of total net productivity (TNP) (Ghoshal and Singh,1995a). Weeds are generally not removed and remain in the fieldafter crop harvest, acting as an important organic input to soil. TheTNP of weeds and crop constitutes the total biological productivityof a system. A large portion of total biological productivity is har-vested from agroecosystems leaving behind stubbles and roots ofcrops and weeds. These plant inputs (TNP of weeds and a part ofcrop TNP), not generally estimated, may play a major role in avail-ability of nutrients to following crops and in the regulation of soilorganic matter dynamics.
It has been reported that herbicides have the potential to vari-ably affect soil organic matter dynamics, especially soil microbialbiomass (Perucci et al., 2000; Vischetti et al., 2002). Enhancement
in soil fertility can be linked to soil microbial biomass. The extentand magnitude of influence of herbicide application on soil biolog-ical properties in general and particularly of soil microbial biomassis scanty and often conflicting (Vischetti et al., 2002; Yang et al.,2007). In response to herbicide application, soil microbial biomass
ay decrease (Wardle and Parkinson, 1990; Macur et al., 2007),ncrease (Debnath et al., 2002; Moreno et al., 2007) or remain unaf-ected (Wardle and Parkinson, 1990; Lupwayi et al., 2007). Impactf herbicide application alone may be different from that of com-ined application of herbicide with soil amendments, dependingpon resource quality. Soil amendments may interact with herbi-ide applications, resulting in changes in soil microbial biomass, yetew studies have been conducted to evaluate such interactions.
Soil microbial biomass is influenced by variations in the qual-ty and quantity of inputs added to soil, either through plantroductivity or exogenous soil amendments. Studies dealingith relationships between plant productivity and soil microbial
iomass have been limited (Broughton and Gross, 2000; Kushwahand Singh, 2005), and often restricted to either aboveground orelowground crop productivity. How total biological productivityffects soil microbial biomass in response to herbicide applicationlone or in combination with various soil amendments remains toe discovered.
We hypothesized that by maintaining higher total biologi-al productivity through higher weed and crop biomass greateroil microbial biomass would be supported. The specific objec-ive under this rice–barley cropping sequence was to estimate theffect of herbicide application either singly or in combination withxogenous soil amendments having contrasting chemical compo-ition on: (1) annual TNP of crop and weeds, (2) variation in soilicrobial biomass concentration, and (3) the role of total biological
roductivity in regulating soil microbial biomass.
. Materials and methods
.1. Experiment site
This study was conducted in the experimental field of theepartment of Botany, Banaras Hindu University at Varanasi
251180 N lat. and 83110 E long.) in India from June 2005 toune 2007. The cropping sequence was rice (Oryza sativa var.DR 97)–barley (Hordeum vulgare var. Lakhan)–summer fallow.reatments were application of recommended dose of herbi-ide as Butachlor (2 kg a.i. ha−1) either singly or in combinationith exogenous soil inputs with contrasting resource quality. All
pplication rates of soil amendments were adjusted to supply0 kg N ha−1. High-quality organic resource was applied in form ofarmyard manure as animal manure (C:N 28) and Sesbania aculeatahoot as green manure (C:N 16), while low-quality organic resourceas applied in form of wheat (Triticum aestivum) straw as crop
esidue (C:N 83). Treatments were: (1) control (no inputs), (2) her-icide only, (3) herbicide + chemical fertilizer (N, P and K in form ofrea, single superphosphate and muriate of potash, respectively),4) herbicide + animal manure, (5) herbicide + green manure and (6)erbicide + crop residue. All treatments were applied once in eachnnual cycle, 1 or 2 days before the sowing of rice. Fresh greenanure and air-dried crop residue were cut into pieces of 2–3 cm.ll soil amendments were incorporated into soil to 5–10 cm depth.erbicide was uniformly sprayed onto soil once in an annual cycle
ust 2–3 days after sowing of rice. Each treatment was replicatedhree times in a randomized block design. Plots had dimensions ofm × 4 m and were separated from one another by a strip of 1 m.
Rice was sown in July and harvested in October. This period coin-ided with the rainy season, which is warm and humid with highelative humidity. Long-term mean temperature varies between 24
nd 34 ◦C. About 85% of the total annual rainfall (1100 mm) occursuring this period. Barley was grown between October and March.ost of the barley period is represented by dry, cold winter season.
he long-term mean monthly temperature during winter rangesetween 10 and 25 ◦C. The field was left fallow after harvest of bar-
and Environment 137 (2010) 241–250
ley from April till June. Summer is dry and hot with mean monthlytemperature of 30–44 ◦C. Rainfall was the only source of moistureand no irrigation was applied. Plots were prepared for seeding ofrice and barley with manual hoeing to 15 cm depth. Each crop washarvested, leaving behind crop stubble and weeds, which wereincorporated into soil prior to sowing of the next crop. The above-ground portion of crop left after crop harvesting was consideredcrop stubble.
The soil was an inceptisol having sandy loam texture and palebrown color. At the beginning of the experiment, soil sampled fromthe control plot had pH of 6.8, bulk density of 1.36 g cm−3, waterholding capacity of 41.1%, organic C of 0.62% and total N of 0.07%.
2.2. Soil sampling and analysis
During each crop cycle, soil was sampled at the seedling, grain-forming and maturity stages and once during summer fallow,totaling seven samplings in each annual cycle. At each sampling, soilcores (5 cm diameter) from a depth of 10 cm were collected fromfour random sites in each replicate plot. Soil samples were thenmixed and passed through a 2 mm sieve. Microbial biomass C andN were estimated by the chloroform fumigation extraction methodusing purified CHCl3 treatment (Brookes et al., 1985; Vance et al.,1987; Singh et al., 2007). Soil microbial biomass C was calculatedby using the equation: MBC = 2.64EC, where EC is the differencebetween organic C extracted from the K2SO4 extracts of fumi-gated and non-fumigated soils. Microbial biomass N was calculatedby using the equation: MBN = EN/0.54, where EN is the differencebetween the amount of N extracted from the K2SO4 extract of fumi-gated and non-fumigated soil and 0.54 is the fraction of biomassN extracted after chloroform fumigation. All soil chemical resultswere the means of triplicate analyses expressed on an oven-driedbasis.
2.3. Total biological productivity
Crop biomass was measured at vegetative, grain-forming andmaturity stages in both crop seasons. This corresponded to 30 and35, 68 and 72, and 105 and 112 days after sowing in first and sec-ond years, respectively, for rice and 38 and 45, 86 and 92, and131 and 138 days after sowing for barley. In each replicate plot,aboveground parts of crop and weeds were harvested close to theground from two randomly located subplots (each 25 cm × 25 cm)and expressed in terms of oven-dry weight. Crop yield (rice grainwith husk and barley seed) was estimated at crop maturity. Theaboveground portion of crops left after harvest was clipped closeto the soil, thoroughly cleaned with a brush, and oven-dry weightestimated to represent crop stubble. For estimation of belowgroundbiomass, two soil monoliths (0–10 cm) were excavated with thehelp of soil corer (10-cm diameter) at each site from where above-ground biomass had been sampled and washed with a fine jet ofwater over a twin sieve assembly (2 mm mesh above and 0.5 mmmesh below). Retrieved roots were air dried, cleaned and separatedinto crop and weed roots. Sum of successive increments in shootsand root biomass were designated aboveground net productivity(ANP) and belowground net productivity (BNP), respectively, ofcrop and weeds. Total net productivity (TNP) was computed as thesum of ANP and BNP of crop and weeds. Sum of crop stubble, crop
roots (roots at grain-forming stage) and weed TNP was consideredplant input. Sum of TNP for crop and weeds in each crop period wasconsidered total biological productivity for that crop period. Sumof total biological productivity for the two cropping seasons withina year was the annual total biological productivity.
P. Singh, N. Ghoshal / Agriculture, Ecosystems and Environment 137 (2010) 241–250 243
Fig. 1. Effect of application of herbicide alone and in combination with soil amend-ments on total net productivity (kg ha−1 crop cycle−1, mean ± S.E.) of crop and weedsof rice and barley cropping periods of two annual cycles; the durations of rice andbarley crop is indicated on top. Values are means ± S.E. CO: control; HC: herbi-cHe
2
meftotft
3
3
aa
Fig. 2. Variations in annual plant input (kg ha−1 year−1, mean ± S.E.) to soil due toaddition of various soil amendments. Values are means ± S.E. CO: control; HC: herbi-cide; HC + CF: herbicide + chemical fertilizer; HC + AM; herbicide + animal manure;
ide; HC + CF: herbicide + chemical fertilizer; HC + AM: herbicide + animal manure;C + GM: herbicide + green manure; HC + CR: herbicide + crop residue; S.E.: standardrror.
.4. Statistical analysis
Data were analyzed using SPSS (version 10.0) package on aicrocomputer. All values were expressed as mean ± one standard
rror. Treatment means were compared using the least square dif-erence (LSD). Differences between treatments and season wereested using two-way analysis of variance (ANOVA). Significancef difference was indicated at p < 0.05 and p < 0.01. Pearson statis-ical tests were performed to test correlations between differentractions of total biological productivity and soil fertility parame-ers.
. Results
.1. Crop productivity
Application of herbicide only and in combination with soilmendments affected ANP, BNP, and consequently TNP, of ricend barley crops. Across all treatments during the 2 years, TNP
HC + GM: herbicide + green manure; HC + CR: herbicide + crop residue; S.E.: standarderror.
of crop ranged from 6379 to 10,953 kg ha−1 for rice and 4067 to6388 kg ha−1 for barley (Fig. 1). About 83–92% and 84–90% of TNPwas allocated to ANP in rice and barley, respectively, across treat-ments. Consistent increase in ANP occurred until crop maturation.However, BNP increased until the grain-forming stage and thendecreased thereafter to crop maturity. Crop stubble constitutedabout 7–15% of crop ANP across treatments during each year. Thetrend of TNP followed that of ANP, as the latter was a major fractionof the former. Higher rice TNP was found with herbicide applicationand amendments (except the herbicide + crop residue treatment)than with herbicide only. The trend was herbicide + chemicalfertilizer (10,803 and 10,753 kg ha−1 crop cycle−1 in the firstand second years, respectively) followed by herbicide + greenmanure (10,068 and 10,303 kg ha−1), herbicide + animal manure(9076 and 9284 kg ha−1), herbicide only (8052 and 8060 kg ha−1),herbicide + crop residue (7748 and 8170 kg ha−1) and controlplots (6376 and 6637 kg ha−1). During barley cropping, TNPwas comparable among control (4067 and 4187 kg ha−1), herbi-cide + chemical fertilizer (5125 and 5382 kg ha−1) and herbicideonly (4556 and 4631 kg ha−1), but was higher in herbicide + animalmanure (6100 and 6388 kg ha−1), herbicide + crop residue (5712and 5944 kg ha−1) and herbicide + green manure (5513 and5671 kg ha−1) treatments. TNP of both crops together was higherin herbicide + chemical fertilizer (15,928 and 16,135 kg ha−1),herbicide + green manure (15,581 and 15,971 kg ha−1), and her-bicide + animal manure (15,176 and 15,672 kg ha−1) than in
herbicide + crop residue (13,460 and 14,114 kg ha−1), herbicideonly (12,608 and 12,691 kg ha−1), and control plots (10,446 and10,823 kg ha−1).
244 P. Singh, N. Ghoshal / Agriculture, Ecosystems and Environment 137 (2010) 241–250
Fig. 3. Variations in levels of soil microbial biomass C and N (�g g−1 dry soil, mean ± S.E.) through two annual cycles in plots receiving herbicide alone; herbicides withf winga
3
bc7tfi3apchrbtmicct
ertilizer and three organic amendments, these additions were made just prior to sos R, B and SF, respectively, day indicates period after sowing of first rice crop.
.2. Weed productivity
Unlike crop TNP, weed TNP was highest in control plots ofoth rice and barley periods (Fig. 1). Across all treatments, allo-ation to TNP of ANP in weeds was 83–89% during rice and5–81% during barley. Maximum weed TNP was found in con-rol plots during rice (8890 and 8703 kg ha−1 crop cycle−1 in therst and second years, respectively) and barley periods (3920 and773 kg ha−1). Weed TNP was reduced considerably with herbicidepplication during both rice (3297 and 3203 kg ha−1) and barleyeriods (2703 and 2583 kg ha−1). During the rice period, herbi-ide application combined with organic amendments resulted inigher weed TNP than herbicide only, except in herbicide + cropesidue treatment. During the barley period, application of her-icide + crop residue significantly increased weed TNP comparedo all other treatments, except herbicide + animal manure treat-
ent. Sum of weed TNP during rice and barley periods was highestn control plots (1281 and 1248 kg ha−1). Herbicide applicationombined with amendments significantly increased weed TNPompared to herbicide only, except in herbicide + crop residuereatment.
of rice; the durations of rice and barley crop and summer fallow is indicated on top
3.3. Total biological productivity
Total biological productivity (sum of crop TNP and weedTNP) during the rice period of both years was higher withherbicide application combined with amendments than withherbicide only, except in herbicide + crop residue (Fig. 1). Dur-ing rice cropping, total biological productivity was highest inherbicide + chemical fertilizer followed in decreasing order byherbicide + green manure, control, herbicide + animal manure, her-bicide only, and herbicide + crop residue treatments. During barleycropping, lowest total biological productivity occurred with herbi-cide only and highest with herbicide + animal manure treatment.Yearly total biological productivity was higher in herbicide + greenmanure (24,438 and 24,607 kg ha−1), herbicide + chemical fertilizer(24,350 and 24,238 kg ha−1), herbicide + animal manure (23,309and 23,509 kg ha−1), and was comparable to control plots (23,256
and 23,299 kg ha−1) whereas it was lower in herbicide + cropresidue (19,298 and 19,911 kg ha−1) and herbicide only (18,608 and18,478 kg ha−1) treatments.
Yearly plant input to soil was maximum in control plots (15,145and 15,086 kg ha−1) and minimum in herbicide only plots (8716
P. Singh, N. Ghoshal / Agriculture, Ecosystems and Environment 137 (2010) 241–250 245
Table 1Variation in soil microbial biomass C (�g g−1 soil ± S.E.) during various crop and fallow periods through two annual cycles; values for rice and barley crops are mean of threesamplings during each crop cycle (2005–2007).
O: control; HC: herbicide; HC + CF: herbicide + chemical fertilizer; HC + AM: herbicidn each row values having different superscripts are significantly different from eac
nd 8744 kg ha−1) (Fig. 2). Yearly plant input to soil was similarmong all herbicide application combined with amendments treat-ent, except herbicide + crop residue.
.4. Crop yield
Across all treatments, crop yield of rice was higher thanarley (Fig. 1). Crop yield of rice was greater with applica-ion of herbicide + chemical fertilizer (1740 and 1810 kg ha−1 inrst and second years, respectively), herbicide + green manure1717 and 1787 kg ha−1) and herbicide + animal manure (1367 and487 kg ha−1) than with herbicide only (1037 and 1127 kg ha−1),erbicide + crop residue (967 and 1052 kg ha−1), and control plots857 and 937 kg ha−1). Crop yield of barley was greater witherbicide + animal manure (900 and 987 kg ha−1), herbicide + cropesidue (840 and 937 kg ha−1), and herbicide + green manure (807nd 880 kg ha−1) than with herbicide + chemical fertilizer (723 and80 kg ha−1), herbicide only (670 and 697 kg ha−1), and controllots (590 and 637 kg ha−1). Notable was the significantly greaterarley yield with herbicide + crop residue input following compara-le yield to that of the control earlier during rice cropping. Likewise,ice yield with herbicide + chemical fertilizer was high, but barleyield in this treatment was similarly low as that of the control.
Total crop yield (i.e., rice yield + barley yield) was greaterith herbicide + green manure (2524 and 2667 kg ha−1), her-
icide + chemical fertilizer (2463 and 2590 kg ha−1), and her-icide + animal manure (2267 and 2473 kg ha−1) than witherbicide + crop residue (1807 and 1988 kg ha−1), herbicide only1707 and 1824 kg ha−1), and control treatments.
.5. Soil microbial biomass C and N
Marked seasonal variations in soil microbial biomass C occurredFig. 3). Within crop cycles and across treatments, levels dippedonsiderably from seedling to grain-forming stage and increasedhereafter towards crop maturity. A general pattern of increasen concentration of microbial biomass C from rice to barley toummer fallow was observed in all treatments, except herbi-ide + green manure treatment. In this treatment, levels decreasedrom rice period to barley period and then increased at summerallow (Table 1). Two-way ANOVA indicated that treatments had
more pronounced effect on soil microbial biomass C (f = 213.5,f = 5, p < 0.05) than cropping period (f = 122.4, df = 6, p < 0.05)
r interaction between treatments and cropping periods (f = 3.3,f = 30, p < 0.05). Across crop cycles and treatments, herbicidepplication combined with amendments had higher microbialiomass C concentration than the control and herbicide only treat-ents. During rice cropping, soil microbial biomass C was greatest
255 ± 11.7c 279 ± 11.0c 226 ± 12.0b 31
imal manure; HC + GM: herbicide + green manure; HC + CR: herbicide + crop residue.r.
with herbicide + green manure (278 and 285 �g g−1 dry soil dur-ing first and second years, respectively), followed in decreasingorder by herbicide + animal manure, herbicide + crop residue, her-bicide + chemical fertilizer, control plots, and lowest in herbicideonly (114 and 110 �g g−1). A similar treatment trend on soil micro-bial biomass C was observed during barley cropping, summerfallow, and across the year.
Seasonal trends in microbial biomass N were similar to thoseof microbial biomass C (Fig. 3). Two-way ANOVA of soil microbialbiomass N data indicated greater influence of treatments (f = 133.1,df = 5, p < 0.05) than of cropping period (f = 40.4, df = 6, p < 0.05)or interaction between treatments and cropping periods (f = 2.3,df = 30, p < 0.05). Soil microbial biomass N ranged from 10.4 to39.4 �g g−1 during rice cropping, 15.9 to 36.0 �g g−1 during barleycropping, and 22.8 to 38.7 �g g−1 during summer fallow.
The C to N ratio of soil microbial biomass was greater withherbicide + animal manure (8.7), herbicide + crop residue (8.7) andherbicide + green manure (8.6) than with herbicide only (7.9) andherbicide + chemical fertilizer (7.5) treatments where it was com-parable to control.
Variations in total crop yield were strongly correlated with theamount of microbial biomass C (r2 = 0.66) and N (r2 = 0.74) acrossall treatments through the annual cycle (Fig. 4A and B). Total bio-logical productivity accounted for 28% and 40% of the variation inmicrobial biomass C and N, respectively, across all the treatments(Fig. 4C and D). Total plant input to soil explained 96% and 86% ofthe variation in soil microbial biomass C and N, respectively, whenno additional nutrients were added, i.e. in control and herbicideonly plots (Fig. 4E and F). When soil amendments were applied,total plant inputs explained only 15% and 53% of the variation insoil microbial biomass C and N, respectively (Fig. 4G and H).
4. Discussion
4.1. Influence of soil amendments on biological productivity
In most weed management strategies, more attention has beenpaid to crop yield than to biological productivity of crops and weeds.Weed management strategies have generally involved applicationof herbicides alone, whereas little information is available whenherbicides are applied in combination with various soil amend-ments. Butachlor, as used in our study, is extensively applied asa pre-emergence or early post-emergence herbicide to inhibit cell
division in emerging seedlings (Rao, 1999). In our study, herbi-cide application inhibited the emergence and growth of weeds, asreflected by lower weed TNP (Fig. 1). Lack of weeds provided lowercompetition for the various natural resources needed by crops, andconsequently, facilitated higher crop TNP and crop yield. Although
246 P. Singh, N. Ghoshal / Agriculture, Ecosystems and Environment 137 (2010) 241–250
Fig. 4. Relationships among annual total biological productivity and its fractions (kg ha−1 year−1, mean ± S.E.) with annual soil microbial biomass C and N (�g g−1 dry soil,mean ± S.E.) through two annual cycles. A: relationship between total crop yield and annual microbial biomass C across the treatments; B: relationship between total crop yielda nual tD iomam weent ass Ca
t2bospMb1macldc
nd annual microbial biomass N across the treatments; C: relationship between an: relationship between annual total biological productivity and annual microbial bicrobial biomass C across the control and herbicide treatments; F: relationship bet
reatments; G: relationship between annual plant input and annual microbial biomnnual microbial biomass N across the combined treatments.
he half life of butachlor is less than a month in soil (Pal et al.,006), its residual effect was also manifested in the subsequentarley period, as evidenced by low weed TNP. Due to applicationf butachlor and ethoxy sulfuron on a silty clay soil in Pakistan, aignificant decrease in weed biomass (30–57%) and an increase inaddy grain yield (10–33%) were observed by Baloch et al. (2005).ukherjee and Singh (2005) found several fold reduction in weed
iomass with application of several herbicides and an increase of4–88% in grain yield of rice. Among all treatments in our study,aximum weed TNP was found in control, since no inhibitory
gents in form of herbicide was applied. Greater weed TNP in theontrol probably competed strongly with crops and resulted inow TNP of crops. Greater total biological productivity was foundespite low TNP of rice and barley in control plots than with appli-ation of herbicide only. Therefore, the increase in crop TNP in
otal biological productivity and annual microbial biomass C across the treatments;ss N across the treatments; E: relationship between annual plant input and annualannual plant input and annual microbial biomass N across the control and herbicideacross the combined treatments; H: relationship between annual plant input and
herbicide treated plots was not proportional to that of the reductionin weed TNP. Total biological productivity of a system is generallyconsidered an index of soil fertility. Hence, it may be inferred thatthe long-term use of herbicide only without other amendmentscould reduce soil fertility, despite achievement of greater crop yield.
In our study, in which soil amendments with equivalent amountof N but varying resource quality were supplied along with her-bicide application, allocation of TNP in crop and weeds followeda pattern that was different from herbicide only, probably due tovariation in availability of nutrients with exogenous soil amend-
ments (Fig. 1). Greater availability of soil nutrients can stimulategrowth of crops (Tollenaar et al., 1994) or of weeds (Cralle et al.,2003). Cathcart et al. (2004) observed that N level could influencesusceptibility of different weed species to herbicides, and therebyweed community structure.
P. Singh, N. Ghoshal / Agriculture, Ecosystems and Environment 137 (2010) 241–250 247
Table 2Variation in soil microbial biomass N (�g g−1 soil ± S.E.) during various crop and fallow periods through two annual cycles; values for rice and barley crops are mean of threesamplings during each crop cycle (2005–2007).
Annual 19.5 ± 0.8a 17.9 ± 0.7a 27.6 ±O: control; HC: herbicide; HC + CF: herbicide + chemical fertilizer; HC + AM: herbicid
n each row values having different superscripts are significantly different from eac
Generally, nutrients are immediately available with applica-ion of chemical fertilizers. Nutrients from chemical fertilizer evenhen applied with herbicide were probably utilized immediately
y crop and weeds during the rice period, as reflected by greaterNP of crop and weeds compared to herbicide only application.ichards (1993) found that a lower dose of herbicide with a lowerose of N resulted in higher weed biomass than crop biomass.im et al. (2006) reported a greater reduction in weed biomass
n response to full herbicide application with increasing N fertilizernputs, suggestive of increased herbicide efficacy with increased
dose. In our study, maximum total biological productivity wasound during rice period with herbicide application + chemical fer-ilizer, because of highest crop TNP and higher weed TNP. Howeveruring the barley period, lowest total biological productivity wasound among treatments with herbicide combined with amend-
ents in the herbicide + chemical fertilizer treatment, probablyecause of limited availability of nutrients. Nutrients supplied toice were either taken up fully by rice or were lost from the systemater on (e.g. leaching).
In herbicide + green manure treatment, high total biologicalroductivity during rice period was probably due to copious avail-bility of nutrients through rapid decomposition of Sesbania shootsuring this period (Singh et al., 2007). During the subsequent barleyeriod, low total biological productivity was likely due to lim-
ted availability of residual nutrients. Singh et al. (2004) reportedo residual effect of nutrient availability from Sesbania, whereasulakh et al. (2000) found considerable availability of residualutrients during a second crop.
Farmyard manure is considered a soil amendment that releasesutrients slowly following application. Ghoshal and Singh (1995a)howed that application of farmyard manure to rice in a rice–lentilotation resulted in lower crop TNP during the rice period thanuring the lentil period. Ramamurthy and Shivshankar (1996) alsoound that nutrients in farmyard manure were not fully available tohe immediate crop, but rather to later crops. Ashiono et al. (2005)bserved that a single application of farmyard manure at the begin-ing of an experiment resulted in greater yield of sorghum for threeonsecutive seasons due to slow mineralization on a sandy loam inenya. In our study, nutrient release from farmyard manure waslow in the early phase (i.e., the rice period), but significant andicked up during the later phase (i.e., the barley period). Carry-overenefit of nutrients was pronounced during barley growth, therebyxplaining the occurrence of maximum total biological productiv-
ty during barley in this treatment.
Contrary reports are also available to suggest little residual effectf manure on crop productivity (Minhas et al., 1994). Availabilityf nutrients in animal manure is considered to be lower than thatf chemical fertilizer (Lithourgidis et al., 2007) and green manure
29.4 ± 1.4b 33.0 ± 1.3c 26.1 ± 0.9d 2.8
imal manure; HC + GM: herbicide + green manure; HC + CR: herbicide + crop residue.r.
(Singh et al., 1994). Total annual crop yield was reported to be simi-lar (Matsi et al., 2003), lower (Griffin et al., 2002) or higher (Ghoshaland Singh, 1995a) due to application of farmyard manure comparedto that of chemical fertilizer. In our study, total crop yield was simi-lar in herbicide + animal manure and herbicide + chemical fertilizertreatments.
Application of crop residues can initially immobilize nutrients,causing nutrient deficiency with crop residue incorporation, butwith greater availability in later crop phases (Singh et al., 2007). Inour study, herbicide + crop residue resulted in lowest total biologi-cal productivity during the rice period, likely due to immobilizationof nutrients. Wheat straw can also suppress the emergence andgrowth of plants (Jodaugiene et al., 2006) due to poor seed to soilcontact (Morris et al., 2009). Doring et al. (2005) found no significantweed reduction with application of straw mulch. Utilization of min-eralized nutrients during the barley period might have increasedcrop and weed TNP, such that total biological productivity and totalcrop yield became comparable to that with herbicide only treat-ment, despite distinctly lower total biological productivity duringthe rice period.
Application of herbicide with varying dose of N has beenreported to alter the efficiency of weed control (Kim et al., 2006).In our study, the amount of N added with amendments was equiv-alent; yet weed TNP varied and might have been due to differentherbicide efficacy caused by variations in N availability from soilamendments with different resource quality.
Crop and weed growth have generally been considered to beinversely proportional. Control and herbicide treated plots repre-sented such relationship in this study. However when herbicideswere applied with various amendments in our study, coexistenceof weeds with crops facilitated the growth of crops, as evidencedby higher crop TNP (Fig. 1). This contradiction to the literature mayhave been possible due to crop-weed interaction in terms of emer-gence, growth and productivity, which could have been altered bynutrient availability through exogenous soil inputs in the presenceof herbicides.
4.2. Influence of soil amendments on soil microbial biomass
In addition to its effect on target weeds, herbicide applicationcan also affect non-target soil microorganisms (Perucci et al., 2000;Vischetti et al., 2002). Herbicide impacts may be both negative andpositive and may not affect all soil microorganisms at the same time
(Alexander, 1981).
In our study, soil microbial biomass C and N were adverselyaffected by herbicide application. Butachlor was found to retardrespiration of microorganisms, especially under laboratory condi-tions (Min et al., 2001). Despite reports of short half life of butachlor
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48 P. Singh, N. Ghoshal / Agriculture, Ecosy
n soil, its negative effect on microbial biomass was found evenuring summer fallow (Tables 1 and 2). Degradation of butachlor
s carried out by microbes and the rate of degradation is depen-ent on its concentration and soil type (Pal et al., 2006). It mighte possible that the degraded products of the herbicide are moreoxic than the herbicide itself (Stratton, 1984), thereby interferingith the growth and activities of microbes. Reduction in soil micro-
ial biomass with application of various herbicides was also foundy Perucci et al. (2000) and Vischetti et al. (2002). Vischetti et al.2002) found that herbicides, especially imazamox and benfluralins acetolactate synthase inhibitors, interfered with the physiologyf microbes. Microorganisms may also utilize degraded products oferbicide as a source of energy, C, and other nutrients for cellularetabolism (Debnath et al., 2002; Moreno et al., 2007).In our study, significantly greater soil microbial biomass C and
with herbicide and organic amendments than with herbicidenly suggested no negative impact of herbicide. This may haveeen due to adsorption of herbicide by organic soil amendments,
eading to formation of bound or recalcitrant forms of herbicidend/or reduced availability to microbial attack (Barriuso et al.,997; Abdelhafid et al., 2000).
Decomposition of organic inputs by microbes may have led to ahange in resource quality with time and caused a shift in micro-ial community structure. Fontaine et al. (2003) conceptualizedhat the relative abundance of r- and k-strategists, two classesf microorganisms dependent upon the quality of fresh organicarbon, could lead to changes in the structure of a soil microbialommunity. In our study, when crop residue (wheat straw) wasdded with herbicide, higher levels of microbial biomass C and Nuring the barley and summer fallow periods as compared to theice period was probably due to release of previously immobilizedoil nutrients (Fig. 3). Crop residues having wide C:N ratio decom-ose slowly and may slow down further when added with herbicideHendrix and Parmelee, 1985; House et al., 1987). This perhapsesulted in delayed release of nutrients and coincided with the laterhase of annual cycle. The nutrient limited condition with herbi-ide + crop residue may have stimulated growth and accumulationf k-strategists as compared to r-strategists.
Sesbania, the green manure, decomposes rapidly due to low C:Natio (Mafongoya et al., 1998) and when combined with herbicide,robably released readily mineralizable C and N immediately after
ts addition. As a result of accumulation of soil microbial biomassand N in the early phase and reduction during the later barley
eriod (Fig. 3), r-strategists may have been supported more than-strategists.
Animal manure (farmyard manure) has been reported to havelower nutrient release, despite relatively low C:N ratio (Ghoshalnd Singh, 1995a). Animal manure is generally more resistant tohort term decomposition (Levi-Minzi et al., 1990) due to com-lex N associations. Loss of N in the form of NH3 volatilization maylso occur (Beauchamp, 1983). In our study, herbicide applicationrobably did not interfere with animal manure decomposition. Thevailability of nutrients from the rapidly decomposable fractionf farmyard manure (Sluijsmans and Kolenbrander, 1977) sus-ained microbial biomass in the initial rice period and may havetimulated the growth of r-strategists. In the subsequent barleyeriod, microbial biomass was likely maintained by the releasef nutrients from the slowly decomposable fraction, which mayave facilitated the growth of k-strategists. The long-term resid-al effect of farmyard manure may be attributed to its slowlyecomposable fraction. Perucci et al. (2000) reported a tendency
or greater soil microbial biomass C with farmyard manure whenpplied with rimsulfuron herbicide, but lower when applied withmazethapyr herbicide. They argued that variation in soil microbialiomass was dependent on the composition and dose of herbi-ide.
and Environment 137 (2010) 241–250
With herbicide + chemical fertilizer, low microbial biomasswas probably due to the lack of additional organic C content,despite addition of inorganic nutrients (Fig. 3). Debnath et al.(2002) reported that the addition of chemical fertilizer (NPK)with butachlor substantially increased microorganisms espe-cially aerobic non-symbiotic N2 fixers and phosphate solubilizingmicroorganisms in the rhizosphere of rice over control. Microor-ganisms can derive a part of their energy, C, and nutrients forcellular metabolism from herbicides and degradation products.
Temporal variation in soil microbial biomass has been stud-ied widely in different ecosystems, and yet variations are not wellunderstood (Patra et al., 1990; Hamel et al., 2006; Spedding et al.,2004). There are many reports available indicating a positive impactof organic residues on temporal variation of soil microbial biomass.Franzluebbers et al. (1995) reported that soil microbial biomasschanged along various stages of crop growth, via alteration in thedistribution of organic inputs from rhizodeposition, crop roots andresidues with time and space. Spedding et al. (2004) noticed greatereffect of season than tillage and residue management on soil micro-bial biomass. In our study, similar temporal pattern of soil microbialbiomass C and N among all treatments showed that the effect ofherbicide addition on temporal variation in soil microbial biomassC and N was minimal (Fig. 3). Lower soil microbial biomass at thegrain-forming stage relative to seedling and maturity stage, irre-spective of differences in climatic conditions (hot-humid duringthe rice period and cold-dry during the barley period), may haveresulted from high competition for nutrients by plants (Van Veenet al., 1989). Nutrient uptake by rapidly growing roots during thegrain-forming stage of each crop (reflected from maximum rootbiomass) might have limited the quantity of nutrients available forsoil microbial biomass. Maximum soil microbial biomass in sum-mer fallow was probably from little competition for nutrients byplants, resulting in immobilization of nutrients into soil microbialbiomass (Ghoshal and Singh, 1995b; Singh et al., 2007).
The type of soil amendment can change the composition of soilmicrobiota, which can be reflected in the C:N ratio of microbialbiomass (Hassink et al., 1990). Since herbicides can be sorbed bydifferent organic inputs (Barriuso et al., 1997; Abdelhafid et al.,2000), greater C:N ratio of soil microbial biomass in response toorganic soil amendments in our study occurred despite applicationof herbicide. The C:N ratio of microbial biomass (8.2–8.8) was com-parable to values reported with organic amendments only and noherbicide (Ghoshal and Singh, 1995b; Ocio et al., 1991). Greater C:Nratio indicates a shift towards dominance of a fungal population inthe microbial biomass. A fungal-dominated soil food web supportsless leaky nutrient cycles compared to bacterial-based food webs,and hence, has the potential to conserve nutrients (Coleman et al.,1983). Herbicide application only, however showed a tendency forlower C:N ratio of microbial biomass, which may be inferred as ashift in dominance towards bacteria. Changes in species composi-tion may affect the functioning of ecosystems, since species mayutilize different resources. Long-term studies are needed to verifychanges in ecosystem function.
4.3. Relationship between total biological productivity and soilmicrobial biomass
Long-term sustainability of agroecosystems to balance eco-nomics and the environment is essential (Smith et al., 2007).Economic sustainability in terms of crop yield, however, has his-torically received most attention. Microbial biomass plays an
important role in regulating crop yield, such as reflected by pos-itive correlations between grain yield and microbial biomass C andN under various agro-climatic zones (Yusuf et al., 2009; Kushwahaand Singh, 2005). In our study, significant and positive corre-lation between total crop yield and annual microbial biomass
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P. Singh, N. Ghoshal / Agriculture, Ecosy
and N indicated the significant role of microbial biomass assource of nutrients to crops (Fig. 4A and B). Although there
as been growing interest in understanding the role of ecosys-em productivity in regulating soil fertility (such as soil microbialiomass), only a few studies with basic information of such rela-ionships have been conducted, even in natural ecosystems. Zakt al. (2003) considered plant productivity to be more responsibleor changes in microbial biomass than species richness in temper-te grasslands. Changes in management strategies may influenceoil microorganisms by changing total plant productivity in tem-erate agricultural grasslands (Bardgett et al., 1998; Zak et al.,994).
In our study, significant positive correlation occurred betweenotal biological productivity and soil microbial biomass N acrossreatments, suggesting the former played a role in regulating theatter (Fig. 4D). Such correlations in agroecosystems may be mis-eading, since a large portion of ANP of crops is harvested andot available for providing nutrients and C to the soil microbialiomass. In agroecosystems, large sources of nutrients to the micro-ial biomass are through endogenous plant inputs and exogenousoil amendments. The behavior of agroecosystems thus may differrom that of natural ecosystems. In treatments where no exogenousnputs were added (i.e., control and herbicide), strongly positiveorrelations between endogenous plant inputs and annual soilicrobial biomass C indicated a major role of endogenous plant
nputs in regulating soil microbial biomass C and N (Fig. 4E and). These treatments followed a somewhat similar pattern to thatxpected in natural ecosystems. Increase in plant input via anncrease in total biological productivity in croplands is possiblehrough crop root, residue, and weeds. Weeds, by virtue of theirapid decomposition (Wardle and Lavelle, 1997), provide additionalutrients thereby facilitating the growth of microbial biomass. Soilicrobial biomass, in turn, may contribute to greater total bio-
ogical productivity by utilizing these nutrients. Positive feedbacketween total biological productivity and soil microbial biomass
s likely. Wardle (1995) however, reported negative relationshipetween soil microbial biomass and plant productivity due to com-etition for resources between plant and microbes in grasslands.he relatively weak relationship between endogenous plant inputsnd soil microbial biomass C and N in the treatments with herbi-ide and amendments indicated that soil microbial biomass waslso governed by input of soil amendments (Fig. 4G and H). Nutri-nts added through various soil amendments were much greaterhan nutrients recycled solely by endogenous plant inputs, result-ng in a masking of the effect of endogenous plant inputs on soil
icrobes.
. Conclusion
Herbicide application resulted in an increase in crop yield, yetotal biological productivity (crop and weed TNP) decreased rela-ive to the control. Soil microbial biomass also tended to decreaseith herbicide only treatment. When herbicide applied with
mendments (chemical fertilizer, animal manure, green manure,nd crop residue), total biological productivity increased due toncreases in TNP of crop and weeds. Any adverse effect of her-icide application on soil microbial biomass and total biologicalroductivity may be negated when combined with organic soilmendments. Variation in weed TNP indicated a change in efficacy
f herbicide due to interactions with amendments. Soil microbialiomass and crop yield were greater with herbicide and amend-ent treatments than herbicide only, despite higher weed TNP.chieving greater total biological productivity, even if maintainingigher weed productivity, will facilitate greater accumulation ofoil microbial biomass and hence soil fertility.
and Environment 137 (2010) 241–250 249
Acknowledgements
We wish to thank the Head and the Program Coordinator, Cen-tre of Advanced Study in Botany, for providing laboratory facilities.University Grants Commission, New Delhi, India provided finan-cial support in form of University Research Fellowship to PratibhaSingh. The authors are thankful to Dr A.J. Franzluebbers, NaturalResource Conservation Centre, USDA, Georgia, USA for editing thelanguage of the manuscript.
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