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Agriculture, Ecosystems and Environment 90 (2002) 125137

Comparison of soil N availability and leaching potential, cropyields and weeds in organic, low-input and conventional

farming systems in northern California

D.D. Poudel a,, W.R. Horwath b, W.T. Lanini c, S.R. Temple d, A.H.C. van Bruggen e

a Department of Renewable Resources, University of Louisiana at Lafayette, P.O. Box 44650, Lafayette, LA 70504, USA

b Department of Land, Air, and Water Resources, University of California, Davis, CA 95616, USAc Department of Vegetable Crops, University of California, Davis, CA 95616, USA

d Department of Agronomy and Range Science, University of California, Davis, CA 95616, USAe Plant Sciences, Biological Farming Systems, Wageningen University, Marijkeweg 22, 6709 PG Wageningen, The Netherlands

Received 4 May 2000; received in revised form 28 November 2000; accepted 5 January 2001

Abstract

Increasing dependence on off-farm inputs including, fertilizers, pesticides and energy for food and fiber production in the

United States and elsewhere is of questionable sustainability resulting in environmental degradation and human health risks.

The organic (no synthetic fertilizer or pesticide use), and low-input (reduced amount of synthetic fertilizer and pesticide use),

farming systems are considered to be an alternative to conventional farming systems, to enhance agricultural sustainability

and environmental quality. Soil N availability and leaching potential, crop yields and weeds are important factors related toagricultural sustainability and environmental quality, yet information on long-term farming system effects on these factors,

especially in the organic and low-input farming systems is limited. Four farming systems: organic, low-input, conventional

(synthetic fertilizer and pesticides applied at recommended rates) 4-year rotation (conv-4) and a conventional 2-year rotation

(conv-2) were evaluated for soil mineral N, potentially mineralizable N (PMN), crop yields and weed biomass in irrigated

processing tomatoes (Lycopersicon esculentum L.) and corn (Zea mays L.) from 1994 to 1998 in Californias Sacramento

Valley. Soil mineral N levels during the cropping season varied by crop, farming system, and the amount and source of N

fertilization. The organic and low-input systems showed 112 and 36% greater PMN pools than the conventional systems,

respectively. However, N mineralization rates of the conventional systems were 100% greater than in the organic and 28%

greater than in the low-input system. Average tomato fruit yield for the 5-year period (19941998) was 71.0 Mg ha1 and

average corn grain yield was 11.6 Mg ha1 and both were not significantly different among farming systems. The organic

system had a greater aboveground weed biomass at harvest compared to other systems. The lower potential risk of N leaching

from lower N mineralization rates in the organic and low-input farming systems appear to improve agricultural sustainabilityand environmental quality while maintaining similar crop yields. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Farming systems; Soil mineral N; Plant tissue N; Weeds; Processing tomato; Corn; California

Corresponding author. Tel.: +1-337-482-6163;

fax: +1-337-482-5395.

E-mail address: [email protected] (D.D. Poudel).

1. Introduction

Conventional farming systems and management

practices have been shown to produce high crop

yields, however, the sustainable soil fertility and

0167-8809/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.

P I I : S 0 1 6 7 - 8 8 0 9 ( 0 1 ) 0 0 1 9 6 - 7


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126 D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137

environmental quality of these production systems is

questionable. Conventional farming systems are often

associated with problems such as nitrate leaching and

groundwater pollution (Foster et al., 1986; Black-mer, 1987), degradation of soil structure (Jordahl and

Karlen, 1993), decreased surface infiltration of water

(Logsdon et al., 1993), and pesticide contamination.

In addition, these farming systems are also associ-

ated with decreased levels of total soil N (Drinkwater

et al., 1998; Wander et al., 1994) and total soil C

(Wander et al., 1994) over time. The answers to

problems associated with conventional practices are

alternative cropping systems that increase soil C and

N and leading to less N to escape soils.

The central Valley in the northern California pro-

duces several agricultural crops including processing

tomatoes (Lycopersicon esculentum L.), corn (Zea

mays L.), beans (Phaseolus vulgaris L.), safflower

(Carthamus tinctorius L.), wheat (Triticum aestivum

L.), cotton (Gossypium hirsutum), rice (Oryza sativa

L.) and sunflower (Helianthus annuus L.). The high

production capacity of this region is attributed to in-

tensive irrigation practices, agrochemical inputs and

intensive tillage. Because of the above mentioned

problems of environmental degradation and public

health risk associated with the conventional farm-

ing systems there is a growing interest in alternativefarming systems in the central Valley of California

and elsewhere. Therefore, alternative farming systems

including organic (no synthetic fertilizer and pesti-

cide use) and low-input (reduced amount of synthetic

fertilizer and pesticide use) farming systems, are be-

ing explored as ways to improve overall soil health,

agricultural sustainability, and environmental quality.

Several researchers have indicated considerable ef-

fects of crop rotation and management practices on

soil N availability (Kamimura et al., 1994; Varvel,

1994; Kolberg et al., 1999; Wienhold and Halvorson,1999), crop yields (Turner et al., 1972; Peterson and

Varvel, 1989; Omay et al., 1998), and weed pressure

(Young et al., 1994; Poudel et al., 1998; Daugovish

et al., 1999) in different crop production environments.

Studies on long-term effects of farming systems and

management practices on soil N availability, crop

yields and weeds, especially in an irrigated Mediter-

ranean row-crop production environment are limited.

An understanding of the effect of farming systems

and management practices on soil N availability, crop

yields, and weeds is important in designing manage-

ment strategies that will both increase agricultural

productivity and minimize the risk of environmental

pollution. The specific objectives of this 5-year study(19941998) were: to measure soil mineral N levels

during a cropping season, to assess N mineralization

rate, to assess crop response to weed pressure, and to

measure crop yields for organic, low-input, and con-

ventional farming systems in irrigated field row-crop

production systems in a Mediterranean environment.

2. Materials and methods

2.1. Site description and field experiment

This research was done as a part of a long-term

study called the Sustainable Agriculture Farming

Systems (SAFS) project initiated in 1989 at the

Agronomy Farm of the University of California at

Davis. The SAFS project consists of 11.3 ha of re-

search plots, and the location (3832N, 12147W,

18 m elevation) is characterized by a Mediterranean

climate with most rainfall occurring during the win-

ter months (DecemberMarch), and relatively little

rain during the growing season. Total annual rain-

fall is typically 400500 mm and daytime tempera-ture averages 2334 C during the growing season

(MarchOctober). The soil at the research site is clas-

sified partially as Reiff loam (coarse-loamy, mixed,

nonacid, thermic Mollic Xerofluvents) and partially

as Yolo silt loam (fine-silty, mixed, nonacid, thermic

Mollic Xerofluvents). On average, soil at the SAFS

site contains 350 g kg1 sand, 460 g kg1 silt, and

190gkg1 clay at 030 cm depth. By FAO classifica-

tion system, the soil at the SAFS site is classified as

Mollic Fluvisol.

At the SAFS project, the conventional, organic, andlow-input farming systems with 4-year rotations are

compared with a conventional farming system with a

2-year rotation (Table 1). The 4-year rotations include

processing tomato, safflower, corn, and bean which

follows winter wheat in the conventional 4-year sys-

tem (conv-4), and a bi-culture of oat (Avena sativa L.)

and vetch (Vicia spp.) in organic and low-input sys-

tems. The oatvetch mix is either harvested for seed,

cut as hay, or incorporated as green manure. The

conventional 2-year system (conv-2) is a tomato and


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D.D. Poudel et al. / Agriculture, Ecosystems and Environment 90 (2002) 125137 127

Table 1

Description of farming system treatments in the SAFS project, University of California, Davis

Farming system Description

Organic 4-Year, five crop rotation including processing tomato, safflower, corn and oats/vetch followed by beans in the4th year; winter legume cover crops grown in between tomato and safflower; safflower and corn; and bean and

tomato; composted animal manure and organic supplements are used; no pesticide application

Low-input 4-Year, five crop rotation including processing tomato, safflower, corn and oats/vetch followed by beans in the

4th year; winter legume cover crops grown in between tomato and safflower; safflower and corn; and bean and

tomato; synthetic fertilizer use is reduced by about one-half the recommended rate; some pesticides used, but

60% less compared to conventional system

Conventional 4-year 4-Year, five crop rotation including processing tomato, safflower, corn and wheat followed by beans in the 4th

year; synthetic fertilizer and pesticide use is based on conventionally recommended rates

Conventional 2-year 2-Year, two crop rotation including processing tomato and wheat; synthetic fertilizer and pesticide use is based

on conventionally recommended rates

wheat rotation. More complete descriptions of the re-

search plots and management practices are reported by

other workers (Scow et al., 1994; Temple et al., 1994).

The organic system is managed according to the

regulations of California Certified Organic Farmers

(CCOF), which do not allow the use of synthetic chem-

ical pesticides or fertilizers. Legume cover crops are

grown in between tomato and safflower; safflower and

corn; and bean and tomato in organic and low-input

farming systems. The cover crop is mowed and incor-

porated about 3 weeks prior to planting of the maincrops. Composted manure is broadcast and incorpo-

rated before planting crops in the organic system. The

low-input system includes reduced fertilizer and pes-

ticide applications in addition to legume cover crops

and mechanical cultivation for weed management.

The conventional systems are managed with practices

typical of the surrounding area, which include the use

of synthetic fertilizers and pesticides. Synthetic fertil-

izers are banded. Each farming system has four repli-

cations consisting of all possible crop rotation entry

points, thus resulting in a total of 56 plots, each mea-suring 68 m18 m. These farming system treatments

are laid out in a randomized block split plot design.

A single row of tomato cv. Brigade was direct

seeded (0.4 kg ha1) in the two conventional system

1520 cm high raised beds with seed centered on

the beds. Bed tops were 1 m wide and beds center

to center were 1.52 m apart. In low-input and or-

ganic systems, tomatoes were transplanted (about

20,000 plants ha1) several weeks after conventional

plot seeding. The delay allowed sufficient time for de-

composition of cover crops, and larger tomato plants

minimized early weed competition in these systems.

Pioneer 3162 corn was planted in the center of a

76.2 cm wide bed (about 70,000 seeds ha1). Both the

tomato and corn were machine harvested.

2.2. Soil analysis

In order to assess soil N availability, five compos-

ite soil samples (015 cm depth) were collected ev-

ery 23 weeks during the cropping season in tomatoand corn plots from 1994 to 1998. Composite soil

samples (015 and 1530 cm) were collected from

all 1998 tomato plots on 4 November 1998 to de-

termine soil mineral N levels after crop harvest in

the organic, low-input, and conventional farming sys-

tems. Soil samples were mixed thoroughly and sieved

through a 2 mm mesh screen. A subsample, 510 g of

soil, was placed in 40 ml 2 M KCl in the field, trans-

ported to the laboratory on ice, and extracted for ni-

trate and ammonium (Bremner and Keeney, 1966) on

the same day. These soil extracts were analyzed forNO3-N, and NH4-N with a diffusion-conductivity an-

alyzer (Carlson, 1978).

In order to study the effects of farming systems on

PMN, the 1997 and 1998 tomato plots were selected

for detailed soil sampling and analysis. After tomato

harvest on 26 July 1997, a composite soil sample

(015 cm depth) from 30 separate soil cores from each

organic, low-input, and conventional tomato plot was

collected on 22 September 1997. Samples were stored

at 4 C until processed. The mineralization of soil


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mineral N was determined over a 160-day laboratory

aerobic incubation (Bonde and Rosswall, 1987). The

mineralized NO3-N was extracted with 2.0 M KCl and

then analyzed on a continuous flow analyzer (LachetInstruments, Milwakee, WI) every 1015 days from

0 to 80 days and every 2025 days during reminder

of the long-term incubation. Potential N mineraliza-

tion data were fit to the following single compartment

exponential model to determine PMN and N kinetics

(Stanford and Smith, 1972):

Nm = N0(1 ekt) (1)

where Nm is the amount of N mineralized at time t,

N0 the initial amount of PMN, and kthe rate constant.

A 7-day anaerobic laboratory incubation (Waring

and Bremner, 1964) was conducted to determine PMNon composite soil samples (015 cm) collected four

times during the 1998 cropping season. A composite

soil sample was obtained from 30 cores per plot. Am-

monium N was extracted with 2.0 M KCl and then

analyzed as above.

To determine soil mineral N levels after crop har-

vest, composite soil samples (015 and 1530 cm)

were collected from all 1998 tomato plots on 4

November 1998.

2.3. Plant analysis

In tomato, petiole sampling was done at first bloom

(60% of plants with flowers) crop stage by walk-

ing through each plot in a U-shaped pattern, and

stripping the fourth leaf from the top of 20 tomato

plants, pulling off all leaflets so that only the petiole

remained, and placing petiole and leaflets in separate

bags. Tissue sampling in corn was done at eight-leaf

(V-8) crop stage. The V-8 stage of corn was gener-

ally around the 8th week after planting. At V-8 stage,

10 plants per plot were cut at soil level for nutrienttesting. The following three leaves, the first unrolled

leaf from the top, and the two leaves below that, were

snipped off where they attached to the stalk and were

saved in a bag. Remaining leaves were cut off and

discarded, and the stalk placed in a separate bag.

In organic and low-input systems, aboveground

biomass samples for cover crops were obtained before

their incorporation into the soil. Weeds were sepa-

rated from the fresh cover crop samples. Cover crop,

weed, tomato petiole and corn tissue samples were

dried at 6065 C, ground to pass through 40-mesh

screen of a Wiley Mill, and submitted to the Univer-

sity of Californias Division of Agriculture and Natu-

ral Resources (UC DANR) Analytical Laboratory foranalyses. Tomato petiole and corn stalk samples were

analyzed for NO3-N (Carlson et al., 1990), and cover

crop and weeds aboveground biomass samples were

analyzed for total-N using combustion gas analysis

(Pella, 1990a,b).

2.4. Statistical analysis

Farming system effects on crop yields were ana-

lyzed by two-way analyses of variance (SAS, 1994).

Tissue N, crop yield, soil mineral N level and weed

biomass in different systems were compared byStudentNewmanKeuls (SNK) multiple range test.

The N mineralization data from 160-day aerobic in-

cubation were fit to Eq. (1) by nonlinear regression

using Marquardt method in SAS (SAS, 1994).

3. Results and discussion

3.1. N inputs

The amount and form of N application to toma-toes and corn varied by the farming systems

(Table 2). In addition to 1.3 kgN ha1 as starter

fertilizer, the organic tomatoes received, on aver-

age, 308 kgN ha1 (190kgNha1 as composted

manures and 118 kgN ha1 as aboveground cover

crop residue), while the low-input and conventional

tomatoes received 198 kg N ha1 (95kgNha1 as

synthetic fertilizers and 103 kg N ha1 as above-

ground cover crop residue) and 165.3 kg N ha1 (syn-

thetic fertilizers), respectively, during 19941998.

Average N application rates (19941998) for theorganic, low-input and conventional corn were

443kgNha1 (292kgNha1 as composted ma-

nures and 151 kg N ha1 as aboveground cover crop

residue), 260 kg N ha1 (106kgNha1 as synthetic

fertilizers and 154 kg N ha1 as aboveground cover

crop residue), and 231 kg N ha1 (synthetic fertiliz-

ers), respectively.

The type, quality, and amount of N additions

and pest control are major management factors that

differentiate the organic farming systems from the


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Table 2

Amount of N applied in tomato and corn crops at SAFS project, Davis (19941998)

Organic (kg ha1) Low-input (kg ha1) Conv-4 fertilizer

(kg ha

1

)

Conv-2 fertilizer

(kg ha

1

)Manurea Amendmentsb Cover cropc Fertilizerd Cover crop

Tomato

1994 123.2 2.8 110 100.8 111 168.0 168.0

1995 108.6 1.0 81 100.8 67 179.2 179.2

1996 308.0 0.4 115 78.4 136 147.8 147.8

1997 226.2 1.0 148 95.2 80 165.7 165.7

1998 181.4 1.0 136 100.8 120 165.7 165.7

Corn

1994 245.3 239 96.1 264 230.7 NAe

1995 175.8 86 141.1 85 230.7 NA

1996 371.8 191 96.3 174 230.7 NA

1997 315.8 116 96.3 121 230.7 NA

1998 350.6 124 97.0 128 231.4 NAa Composted poultry manure applied before planting.b Fish powder, sea weed applied at the time of planting.c N in aboveground biomass. Contribution from weeds is included.d Synthetic fertilizer applied at the time of planting (small amount as starter fertilizer) and 45 weeks after planting.e Not applicable.

conventional system. In conventional system, nutrient

imbalance in the soil and pesticide use often impact

agricultural sustainability and environmental quality.

The balance of N, P, and K in the soil and pesticide

use were identified as indicators of agricultural sus-tainability in Costa Rica (Jansen et al., 1995), while

nutrient replenishment (annual amounts of N, P, and

K) was identified as the most important factor related

to the sustainability of conventional vegetable farm-

ing systems in Mindanao, Philippines (Poudel et al.,

1998). At SAFS, the differences in the amount and

source of N in the organic, low-input and conven-

tional farming systems are expected to impact soil

mineral N levels during the cropping season. Several

researchers (Aulakh et al., 1991; Wander et al., 1994;

Franzluebbers et al., 1995) have reported the effects

of type, amount, and form of N application on soilmineral N levels in other parts of the USA.

3.2. Soil mineral N levels

Soil mineral N levels (015 cm depth) during a crop-

ping season differed by crops, amount and sources of

N application, and farming systems (Tables 2 and 3).

Although statistically not significant, average surface

soil mineral N levels for the organic and low-input

tomato systems were lower during all the cropping

seasons compared to the conventional farming systems

(Table 3). In contrast, the organic and low-input corn

systems showed consistently higher surface soil min-

eral N levels during a cropping season compared to the

Table 3

Average soil mineral N (NO3-N+NH4-N) levels (015 cm depth)

during the cropping season for tomato and corn crops in the

organic, low-input and conventional systems at SAFS project,

Davis (19941998) (n = 5)a

Organic

(mg kg1)

Low-input

(mg kg1)

Conv-4

(mg kg1)

Conv-2

(mg kg1)

Tomato

1994 14.2 a 20.9 a 28.5 a 30.1 a

1995 19.3 a 24.1 a 31.0 a 25.7 a

1996 20.9 a 17 a 30.4 a 26.3 a

1997 27.6 a 24.3 a 36.2 a 39.5 a

1998 30.9 a 26.8 a 41.9 a 39.6 a

Corn

1994 42.8 a 28.5 a 14.3 a NAb

1995 23.9 a 20.9 a 17.9 a NA

1996 33.2 a 16.9 a 15.4 a NA

1997 61.9 a 42.1 ab 26.6 b NA

1998 37.9 a 20.8 b 17.4 b NA

a Different letters within a row are significantly different at

0.05 probability level by SNK.b Not applicable.


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conventional system. This suggests that soil mineral N

levels vary by the amount of N application (Table 2),

and crop N uptake pattern. Tomato has the highest N

absorption rate between 40 and 50 days after emer-gence (Dumas, 1990; Tesi and Giustiniani, 1987) with

a rate of 36 kgN ha1 per day (Cavero et al., 1997).

Corn takes up to 75% of its total N after tasseling, the

onset of the reproductive stage (Friedrich et al., 1979;

Mills and McElhannon, 1982). Tasseling generally oc-

curs during the 8th week after planting. Differences in

soil mineral N levels between tomato and corn crops

and among the organic, low-input and conventional

farming systems during a cropping season suggest that

appropriate crop specific N management strategies are

necessary to minimize environmental pollution from

these farming systems.

3.3. Potential N mineralization

The potential N mineralization assay by both

160-day aerobic incubation and 7-day anaerobic incu-

Fig. 1. Potentially mineralizable N assayed by 7-day anaerobic incubation during the 1998 cropping season. Urea (46% N) was sidedressed

on 18 May in low-input and on 20 May after soil sampling in conventional farming systems. Different letters above a column in a group

are significantly different at 0.05 probability as determined by SNK test (n = 4).

Table 4

Average amount of potentially mineralization N and N turnover

rate assayed by 160-day aerobic incubation for organic, low-input

and conventional farming systems at SAFS project, Davisa (n =

4) (different letters within a column are significantly different at0.05 probability by SNK)

Treatment Potentially mineralizable N

(mg NO3-Nkg1)

Turnover rate (rate

constant (k) per day)

Organic 100.5 a 0.0063 b

Low-input 64.3 b 0.0098 ab

Conv-4 46.8 b 0.0137 a

Conv-2 47.8 b 0.0113 ab

a Soil sampled on 22 September 1997; tomato harvested on 26

July 1997.

bation showed greater pools of PMN in the two covercrop-based farming systems (organic and low-input)

(Table 4, Fig. 1). Potentially mineralizable N has

been used as an index of N availability by many

workers (El-Haris et al., 1983; Drinkwater et al.,

1995; Franzluebbers et al., 1995; Vanotti et al., 1995;


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Barrios et al., 1998; Needelman et al., 1999). Results

from 160-day incubation assay showed 112 and 56%

greater PMN pool in the organic system compared to

the conventional and low-input systems, respectively.The in-season PMN levels were also consistently

higher in the organic system, except for the 20 May

spike in the low-input system (Fig. 1), attributable to

the sidedressing of 90 kg N ha1 as urea 2 days be-

fore sampling. Notwithstanding larger PMN pools in

cover crop-based farming systems (Table 4, Fig. 1),

N mineralization rate in the conventional system was

100% greater than in the organic and 28% greater

than in the low-input system (Table 4). The lower

turnover rate in the cover crop-based farming systems

is attributed to the differences in the quality of SOM

which is linked to the chemical stabilization and phys-

ical protection of its labile pool (Ladd et al., 1985;

Drinkwater et al., 1998). Wander et al. (1994) com-

pared biologically active SOM pools for three crop

rotations (organic with cattle manure and cover crops,

cash-grain-based organic cover crops, and a conven-

tional cash-grain-based rotation with mineral fertil-

izer). They found a higher total C and N, particulate

SOM, and reduced water dispersible organic C in

cover-cropped soil and suggested that the SOM pool

of the cover-cropped treatment was more stable than

the SOM in other treatments. The lower N miner-alization rate in the organic and low-input farming

systems corresponds with a greater accumulation of

N in these systems (Clark et al., 1998) and a reduced

risk for N leaching and groundwater pollution. In fact,

soil mineral N levels after crop harvest in the organic

and low-input systems were remarkably lower com-

pared to those in conventional plots at SAFS in 1998

(Table 5) which apparently suggests a reduced risk of

N leaching and groundwater contamination in alter-

native farming systems. Kamimura et al. (1994) also

found increased total soil N values for organic paddytreatments in their long-term field experiment in Japan.

Table 5

Average soil mineral N (NO3-N + NH4-N) levels after 1998 tomato harvest in the organic, low-input and conventional systems at SAFS

project, Davis (n = 4)a

Depth (cm) Organic (mg kg1) Low-input (mg kg1) Conv-4 (mg kg1) Conv-2 (mg kg1)

015 20.19 b 17.17 b 44.10 a 37.59 a

1530 7.11 a 5.86 a 15.30 a 9.84 a

a Different letters within a row are significantly different at 0.05 probability level by SNK.

3.4. Plant tissue N

Effects of the amount of N application and soil

mineral N levels on plant tissues N concentration intomatoes and corn (Table 6) crops were observed in

the organic, low-input and conventional farming sys-

tems. Plant tissue N results indicated that both the

organic tomato and corn crops had N limitation prob-

lem especially during the initial years of this study,

when N application rates in this system were rela-

tively small (Table 2). Petiole NO3-N concentrations at

first bloom crop stage was low in organic tomatoes in

1994 and 1995 (Table 6), which increased remarkably

following threefold increase in the amount of com-

post N application in 1996 in this system (Table 2).

Compost application rates in organic tomatoes de-

clined in 1997 and 1998 but were higher than that

of the 1994 and 1995. Increased amounts of com-

post N application in organic tomatoes was reflected

in higher levels of petiole NO3-N concentrations in

this system in 19961998 (Table 6). Organic corn tis-

sue N also showed a similar relationship with N input.

In 1995, when composted manure application in or-

ganic corn was reduced by 28% compared to the 1994

level (Table 2), stalk NO3-N concentration in the or-

ganic corn were lower compared to the low-input and

conv-4 systems (Table 6). However, when compostedmanure application in organic corn was doubled in

1996 and subsequent years compared to 1995, stalk

NO3-N concentrations between organic, low-input and

conv-4 systems did not differ. These results indicate

that the organic system required a large amount of

N input to meet crop demand and indicated differ-

ences in N mineralization potential between the or-

ganic, low-input and conventional farming systems.

Conventional systems received large amounts of in-

organic N through sidedressing (Table 2) at a time of

high plant demand, while organic systems more likelyobtained smaller amounts of N over a long period of


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Table 6

Average petiole NO3-N for tomato at the first bloom crop stage and stalk NO 3-N for corn at V-8 crop stage at SAFS project, Davis

(19941998) (n = 4)a

Organic(mg NO3-Nkg1) Low-input(mg NO3-Nkg

1) Conv-4(mg NO3-Nkg1) Conv-2(mg NO3-Nkg

1)

Tomato

1994 4267 b 1373 c 14275 a 14850 a

1995 1023 b 8580 a 7920 a 7770 a

1996 7893 b 8840 b 12350 a 11350 a

1997 10875 b 13000 a 8373 c 10575 b

1998 15200 a 13550 ab 9993 bc 7209 c

Corn

1994 7318 a 5808 a 6790 a NAb

1995 682 c 4250 b 7523 a NA

1996 8218 a 7943 a 6793 a NA

1997 4097 a 5038 a 5145 a NA

1998 5213 a 4968 a 5580 a NAa Different letters within a row are significantly different at 0.05 probability by SNK.b Not applicable.

time, from the slow release of N from compost and

cover crops. According to Westerman et al. (1988), po-

tentially about 50% of total available N in composted

broiler and turkey litter is available within a few weeks

after application, while potentially about 60% is avail-

able within 810 months. Several processes such as

mineralization, immobilization, and nitrification affect

the forms and amount of soil N available to crop plants(Reddy et al., 1979; Tisdale et al., 1993), and factors

such as air and soil temperature, transpiration, and soil

water potential affect N uptake and translocation (van

Keulen and van Heemst, 1982). Based on results from

this study, it can be safely stated that a close monitor-

ing of the N status of the plants is necessary especially

in the organic and low-input systems to improve agri-

cultural productivity and environmental quality of an

agroecosystem.

3.5. Crop yields

There were significant farming system and year

interaction effects (ANOVA, p < 0.01) on tomato

fruit and corn grain yields during this study duration.

Tomato fruit yields were significantly different among

the farming systems for three (1994, 1995 and 1998)

out of five study years (19941998) (Fig. 2), while

corn yields showed system effects only for 1994 and

1995 (Fig. 3). Organic tomato yield was lower by 36%

in 1994, 4% in 1995, and 7% in 1997 compared to

conv-2 tomato. Organic tomato had a greater yield by

18% in 1996 and 13% in 1998 than conv-2 tomato. In

1998, tomato fruit yield was lowest in the conv-4 sys-

tem followed by the conv-2 system, as conventional

tomato yield, on average, dropped by 37%, while the

two cover-cropped, transplanted systems dropped by

17% from that of the previous year. The large decrease

for conventional systems was partially the result ofhaving to replant. The initial planting had emergence

problems due to an unusually wet spring, followed by

extreme heat in the summer. Increased organic tomato

fruit yield in recent years are attributed to increased

rates of composted manure application, which appar-

ently have prevented potential N deficiency problems

(Cavero et al., 1997). Although low-input corn grain

yields, on average, were 10% greater than conv-4 corn

yields, yield differences between low-input and conv-4

systems appears to be declining. Low-input corn grain

yields were 27% greater in 1994, 9% in 1995, and15% in 1996 compared to the conventional system,

while they were 1% lower in 1997 and 2% in 1998.

The 1998 low-input corn grain yield was 14% lower

than organic. These results indicate that the manage-

ment practices used in alternative farming system have

potential for producing comparable yields to conven-

tional farming systems. Similar results are reported

by other workers in Pennsylvania (Drinkwater et al.,

1998), in North Carolina (King and Buchanan, 1993)

and in California (Drinkwater et al., 1995; McGuire


9/13


Fig. 2. Tomato fruit yields in the organic, low-input and conventional farming systems at SAFS Project, 19941998. Different letters above

a column in a group are significantly different at 0.05 probability as determined by SNK test (n = 4).

Fig. 3. Corn grain yields in the organic, low-input and conventional farming systems at SAFS Project, 19941998. Different letters above

a column in a group are significantly different at 0.05 probability as determined by SNK test (n = 4).


10/13


et al., 1998). Comparable organic rice yields to con-

ventional farming systems are reported by Kamimura

et al. (1994) in Japan.

Although the organic, low-input and conventionalfarming systems at SAFS showed comparable crop

yields, they have differences in economic viability.

Poudel et al. (2001) reported lower cumulative net re-

turns for the organic system than for the low-input and

conventional farming systems over the last 11 years

at SAFS. Conventional farming system with 2-year

crop rotation was the most profitable system due to

growing tomatoes every other year, while organic sys-

tem with premium prices performed better than conv-4

and low-input system, a net loss occurred with the or-

ganic system without premium prices. Higher produc-

tion costs in the organic system were due to additional

expenses in compost use, planting, cover crop man-

agement, and pest control (Clark et al., 1999). This

suggests that large acreage in organic production will

pull the prices down and eventually loss of farm prof-

itability. It means even if the organic systems are eco-

logically and agronomically sound, economically their

widespread adoption is questionable.

3.6. Weeds

There were significant farming system effects onaboveground weed biomass in tomato in 1996 and

Table 7

Average aboveground weed biomass at harvest in the organic, low-input and conventional farming systems at SAFS project, Davis

(19941998) (n = 4)a

Organic

(dry weight kg ha1)

Low-input

(dry weight kg ha1)

Conv-4

(dry weight kg ha1)

Conv-2

(dry weight kg ha1)

Tomato

1994 150 a 157 a 173 a 141 a

1995 621 a 595 a 366 a 273 a

1996 1550 a 1690 a 467 b 204 b1997 264 a 124 a 278 a 74 a

1998 350 a 67 a 140 a 4 a

Corn

1994 69 a 49 a 79 a NAb

1995 2364 a 998 b 1572 ab NA

1996 454 a 311 a 461 a NA

1997 2354 a 1509 a 599 a NA

1998 c NA

a Different letters within a row are significantly different at 0.05 probability by SNK.b Not applicable.c Data not available.

in corn in 1995 (Table 7). However, percent weed

cover monitored during the cropping seasons showed

more weed pressure in the organic and low-input sys-

tems than conventional systems both in tomato andcorn during all the study years (data not shown). The

greater weed pressure in the organic and low-input

systems was reflected in the cost of weed manage-

ment reported by Clark et al. (1999) in the organic,

low-input and conventional tomato systems at the

SAFS Project. They found the organic or the low-input

tomato systems cost $571 ha1 for weed management

while the conventional systems cost $420 ha1 during

19931996.

Several researchers reported effects of crop rota-

tion on weed pressure. Young et al. (1994) observed

increased winter annual grass weed populations over

time in monoculture wheat (winter wheat and spring

wheat) compared to a 3-year rotation of winter wheat,

spring barley ( Hordeum vulgare L.) and spring dry

pea (Pisum sativum L.) in the Pacific Northwest. Sim-

ilarly, Daugovish et al. (1999) reported the superiority

of 3-year rotations to 2-year rotations in controlling

winter annual grass weeds in Nebraska. We did not

see a difference in weed biomass between the con-

ventional 2-year and the conventional 4-year rotations

in our study. Although a shorter rotation might be

expected to have more weeds, hand weeding in thetomato crop every other year, prevented the buildup


11/13


of weed seed in this system, unlike the other studies

where hand weeding was not used.

More weeds were observed in the organic and

low-input systems where herbicides were either notused or used less frequently. This is in agreement

with Barberi et al. (1998), who observed almost four

times more weed seed in an organic corn system than

in a conventionally managed system.

In this study, crop yields and weed biomass at har-

vest varied. The highest organic tomato yield occurred

in 1996 (Fig. 2) when weed biomass at harvest was

also highest (Table 7). Most of the weed growth was

in the furrow and edges of the bed, which did not im-

pact tomato growth. High rates of compost were ap-

plied in 1996, and weed growth was much greater in

the later part of the growing season, as a result of this

added N. Miyama (1999) observed more late season

weed growth in tomatoes following early weed-free

conditions, when applied N or compost was increased.

In contrast, corn yields in 1995 were lowest when

aboveground weed biomass at harvest was greatest.

Hand weeding is not used in corn, and thus weeds

that emerge early compete with the crop for the full

season. Although not statistically significant, organic

corn yields in 1997 were lower compared to low-input

and conventional treatments when organic corn had a

higher aboveground weed biomass at harvest. Theseresults suggest that more understanding of the ecol-

ogy of weeds and interrelationship between crops and

weed pressure and N application is needed to improve

agricultural productivity of the organic and low-input

farming systems while enhancing environmental qual-

ity of an agroecosystem.

4. Conclusions

Soil N availability and N leaching potential duringa cropping season varied by crop, farming system,

and the amount and source of N application. Cover

crop-based farming systems appear to have a larger

PMN pool, but a slower, more continuous release of

mineral N throughout the growing season, while con-

ventional systems supplied mineral N in pulses as a

result of fertilizer management. Late-season soil min-

eral N levels apparently depended on the amount of

N application and crop removal; and were generally

high both in tomatoes (conventional systems) and in

corn (organic and low-input systems) in the later part

of this study. Therefore, post-harvest N management

strategies appeared necessary to conserve N for future

crop use while minimizing the risk of N leaching andgroundwater pollution in field crop production sys-

tems. Crop response to N application rates and weed

pressure varied by crop. Tomato responded to the

amount of N application, while corn responded more

to weed competition. Tomato has a higher crop value

than corn and therefore hand weeding supplemented

cultivation. This reduction in weed competition re-

sulted in fertility being a more important variable

for yield. Organic and low-input systems generally

had higher amounts of aboveground weed biomass

at harvest indicating that these farming systems had

more weed competition. Application of appropriate

amount of N and development of N management plan

considering crops grown, management practices used,

weed consideration, and N mineralization potential

appears to be a very important factor to improve crop

yield and profitability while minimizing the risk of N

leaching and groundwater pollution.

Acknowledgements

We acknowledge the work of Oscar Somasco, MaryKirk Wyland, Diana Friedman, and Sean Clark, former

research managers on the SAFS Project, and William

Cruickshank, Don Stewart, and Peter Brostrom, SAFS

crop production managers. We are thankful to the stu-

dents, faculty, and staff members from the University

of California at Davis who contributed to research

efforts at the SAFS project during this study. Finan-

cial and/or technical support for the SAFS project

has been provided by several agencies including WR

SARE of the USDA, University of California Sus-

tainable Agriculture and Education Program (UC

SAREP) and University of California Division of

Agriculture and Natural Resources (UC DANR).

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