Managing Soil Food Webs for Enriched and Suppressive Soils: Effects of Cover Crop Diversity and Quality
Soil food webs cycle nutrients and regulate parasites and pathogens, improving agricultural productivity and ecosystems health. This study was conducted to determine if soil food web services and crop yield increase with cover cropping in a conservation tillage system. We investigated the affect of three cover crop treatments (vetch/pea, oat/wheat and oat/wheat/pea/vetch) with low, medium and high C/N and a bare fallow control. The growth of a cover crop was the most important factor affecting nematode communities. Nematode groups associated with increased nutrient cycling were affected by the C/N ratio of cover crops, increasing after nitrogen rich cover crops.
Our primary aim is to provide farmers with management strategies that optimize soil food web services, specifically nutrient cycling and disease suppression.
Objective 1: Understand the effects of cover crop quality on the soil food web. Do cover crop carbon to nitrogen ratios influence the nutrient cycling and suppressive capacity of soils as indicated by the Structure Index (SI) and Enrichment Index (EI)?
Objective 2: Quantify the effects of increased cover crop diversity on the soil food web. Can complex cover crop mixtures increase disease suppressive and nutrient cycling capacity of soil food webs as indicated by the SI and EI?
Project activities were initiated in September 2005 at two field sites following our objectives to 1) Understand the effects of cover crop carbon to nitrogen ratio on the soil food web and 2) Quantify the effects of increased cover crop diversity on the soil food web.
Field site 1 is located at the UC Davis Student Farm. Cover crop treatments were planted in September 2005, consisting of two and four species cover crop mixtures and a no cover crop control.
Treatments are as follows: 1) Cayuse Oats 60lbs/A +Triticale 118 60lbs/A , 2) Magnus Peas 80lbs/A + Purple/Lana vetch 40lbs/A, 3) Cayuse Oats 15 lbs/A + Triticale 118 15 lbs/A + Magnus Peas 60lbs/A +Purple/Lana Vetch 30 lbs/A, 4) Fallow control. Each treatment had five replications in 30 x 40ft blocks. Initial soil samples were taken in August 2005, and we assessed nematode taxa to perform faunal analyses. In May of 2006 we mowed the cover crop, strip tilled and transplanted tomatoes. At the time of cover crop mow down we took cover crop samples for biomass, carbon, and nitrogen levels; and soil samples for total nitrogen, total carbon, NO3, NH4, and nematode faunal analysis. The tomato crop was harvested in September 2006. At the time of harvest tomato plant biomass, red/green tomato harvest weight, and soil for nematode faunal analysis.
Cover crops were planted for year two into undisturbed tomato beds on October 9th-11th, 2006. Cover crop treatments were adjusted as follows: 1) Cayuse Oats 60lbs/A + Wheat 60lbs/A, 2) Magnus Pea 75 lbs/A + Lana/Common vetch 50lbs/A, 3) Cayuse Oats 10lbs/A + Wheat 10lbs/A + Lana/Common Vetch 50lbs/A + Magnus Pea 75 lbs/A, 4) no cover crop control. The grain plus legume mix was planted with legumes alone every other row. Severe pest pressure from slug and rodent populations necessitated spot reseeding of cover crops in year two. In March 2007 cover crop mow-down was followed by strip tillage and direct seeding to corn. No-cover crop control plots were flame weeded periodically during winter months to maintain low biomass levels. Production crops were watered with buried drip. Buried drip maintained a dry soil crust and few weeds germinated in tomato or corn beds.
Soil samples were taken after major field operations in May and September of 2006. In 2007 soil sampling was increased to include samples 3, 7, 14 and 17 weeks after cover crop mow down. Twelve-core composite samples (2.5cm diameter and 15cm deep) were taken from each plot. Cores were collected randomly and the holes closed with minimal soil disturbance. Samples were thoroughly mixed and divided for nematode faunal analysis (350g) and soil moisture (50g). After major field operations soil was also partitioned for soil N, soil C, NH4, NO3 and ph determination.
Cover crop biomass was sampled from two randomly chosen 50-centimeter-square quadrats. Samples were weighed in the field and dried at 60º C. Dry samples were chopped with a hedge trimmer and mixed for subsampling. Subsamples were ground to pass through a 20 mesh screen and oven dried at 50º C.
Tomato harvest for yield was taken from two 10 foot sections. Tomato plants were cut at the base and plants shaken onto tarps to mimic mechanical harvest. Plant biomass, red and green tomatoes were weighed in the field. Remaining tomatoes were harvested and removed from the field site.
Corn silage was sampled from randomly chosen 10 foot row sections of two center beds. The wet weight was measured, and two stalks per sample were randomly separated and dried for moisture and carbon/ nitrogen analysis. Subsamples were field dried for 3 days at mean high temperatures of 90º F and oven dried at 70º C for 4 days. Dry samples were weighed, broken up, mixed and sub-sampled for nitrogen and carbon analysis.
Field site 2 is located in the UC Davis Mesocosm Facility. The Mesocosm Facility provides plots isolated by 2 ft diameter barrels sunken into the ground, filled with a sand based medium and with watering frequency and duration controlled by a timer.
Plots were planted in September 2005 to determine the effects of annual cover crop mixtures, annual monocrops, perennial grass mixtures, and perennial grass monocrops. Treatments including a bare fallow control area replicated four times in a randomized complete block. Cover crops for year one include Lana Vetch, Magnus Pea, Triticale 118, and Cayuse Oats in monoculture and fours species mixtures. Native grasses include Poa secunda secunda, Vulpia microstachys, and Elymus Glaucus in monoculture and three species mixtures. Soil samples were taken for nematode faunal analysis in September 2005, May 2006 and September 2006. Cover crop and cash crop biomass were taken in May and September of 2006 respectively.
In year 2 cover crop plots were replanted. Cover crop combinations were adjusted to increase the number of species in cover crop mixtures. Complementary species were added to the originals treatments: yellow clover, bell bean and wheat. Native grass plots were abandoned due to low establishment in year one. Additional soil samples were taken in April and July 2007. Cover crop biomass was taken in April 2007 and corn silage biomass in August 2007.
Identification and Analysis
All nematode samples were extracted and enumerated following soil sampling. Nematodes were identified to genus and assigned to trophic groups according to Yeates et al (1993a) and colonizer-persister groups based on Bongers (1990), Bongers and Bongers (1998). Soil food web indices including the Structure Index, Basal Index, Channel Index, and Enrichment Index were calculated after Ferris et al. (2001).
The data for field site one at all sampling dates was analyzed as a randomized complete block using the proc mixed ANOVA procedure in SAS (Statistical Analysis Systems Inc. 1989). Trophic group total abundance was log transformed. Correlations, linear and stepwise regressions (SAS) were used to analyze relationships between nematode fauna and soil/ cover crop properties.
Currently, we are preparing publication of the data from field site 1. This publication is in the rough draft stage and will be finalized for submission by the end of February 2008. Further publication of this work is necessary in a format more accessible to producers. I plan to write a producer pamphlet by the end of March 2008. This work will also be presented as part of the University of California at Davis Department of Nematology Seminar series on March 3rd. I will also present this information to growers at the annual Long Term Research in Agricultural Sustainability at UC Davis workshop and other grower oriented venues.
Impacts and Contributions/Outcomes
Evaluation of cover cropping systems for soil food web services reveals that it is important for managers to consider cover crop quality as well as quantity. Cover crops increased nutrient cycling capacity signaled by the enrichment index (EI). However, high biomass producing grain cover crops that increase the EI were associated with low crop yield. In contrast, total number of enrichment opportunist taxa was affected by both quantity and quality of cover crops grown. Monitoring the abundance of enrichment opportunists may provide managers with a new tool to evaluate soil food web nutrient cycling capacity.