Healthy productive crops are intimately dependent on soil organisms that cycle nutrients and regulate plant pests. In field trials at the University of California we used nematode indicators of soil health to evaluate cover crop ability to increase beneficial soil organisms. We hypothesized that cover crop quality (carbon to nitrogen ratio and lignin content) drive differences in soil food web function. Results indicate that although carbon to nitrogen ratios do influence soil organisms the most important factor is the presence or absence of a cover crop. Sites with continuous cover had more highly functioning soil food webs than winter fallow.
Soils are dynamic living interfaces, rich with millions of species of organisms. Diverse, active communities of soil bacteria, fungi, protozoa, nematodes and insects are critical to healthy soil systems and healthy plants. For example, soil organisms rip and tear, digest and decay organic material, providing a biotic fertilizer to feed crops. Other soil biota attack pest organisms, balancing their numbers and protecting crops. In order to maintain high yields and minimize detrimental environmental impact it is critical that we identify ways to manage soil biota for enhanced nutrient cycling. The Millennium Ecosystem Assessment states “Specific forms of (soil) biodiversity are critical to performing the buffering mechanisms that ensure the efficient use and cycling of nutrients in ecosystems” (MEA, 2006b).
This project focused on microscopic worms called nematodes. More than 500 nematodes can be found in just one teaspoon of soil. Diverse and abundant, nematodes tell us about the size and activity of soil biological communities (Bongers and Bongers, 1998).
Cover crops are a useful tool for farmers. Grown before cash crops, cover crops add nitrogen and organic matter to the soil (Snapp et al., 2005). Growing cover crops may also provide an effective tool to regulate soil biota and the ecosystem services they provide. Until recently most attempts to improve soil biological communities have focused on the application of amendments such as manure, compost, sawdust and municipal waste (Yeates et al., 1993b; Bulluck et al., 2002; Okada and Harada, 2007) that may be difficult or expensive for farmers to access, contain large numbers of extant organisms, or heavy metal contaminants (Porazinska et al., 1999). Most cover crop studies that include nematodes have investigated nematode-suppressive green manures such as sorghum sudangrass, rye, and mustards (Abawi and Widmer, 2000; Wang et al., 2006a; Machado et al., 2007, Collins et al., 2006; Everts et al., 2006), or focus only on plant-parasitic nematodes (Kimpinski et al., 2000; Wang et al., 2004). Here we looked at the effect of readily available legume and grain cover crops on nematode indicators of soil health and related plant productivity.
Our primary aim is to provide farmers with management strategies that optimize soil food web services, namely 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 nematode biological indicators including 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?