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Soil Health Responses to Short- and Long-Term Organic Inputs


Over 400,000 acres of California farmland are certified organic and receive all fertility from organic inputs. In addition, due to new legislation requiring the diversion of organic waste from landfills into composting facilities, compost usage is likely to increase even on non-organically managed land. Organic inputs can potentially have many positive effects on soil health, such as increasing the soil organic carbon (C) content, nitrogen (N) cycling for plant growth, and improving the resilience of soil functions to stressful events. However, they may also have negative effects: they can be expensive, and inappropriate application may lead to poor crop growth or environmental pollution. To manage organic inputs in such a way as to maximize the benefits and reduce the risks, California growers need good region- and system-specific information on the effects of a range of organic inputs on the many facets of soil health. The goal of this study is to provide research-based information about the effect of several organic inputs in irrigated California cropping systems on different aspects of soil health, especially as they relate to soil N cycling, C accumulation, and biological function. A secondary focus is to identify important sources of variability both within those effects and within metrics themselves, as soils, amendments, and cropping systems are all intrinsically variable, and “one-size-fits-all” recommendations may not be appropriate. One goal of applying organic materials is short-term N fertility, but there is uncertainty about how much N mineralization can be expected. I used lab experiments to measure the range and variability of net N mineralization from different organic amendments and compost types in common use in California, and tested how this mineralization is affected by soil management history, and how it can be predicted by simple biochemical measurements. I found that materials had a wide range of N mineralization potentials, from immobilization to almost 100% availability, and that availability was broadly predicted by the amendment C to N ratio. Soil management history did not affect the N release rate. Another potential short-term effect of organic inputs is to buffer the soil community against shocks. My second experiment explored whether a recent compost addition could increase the resilience of microbial C and N cycling functions to a sudden chemical stress, the addition of elemental sulfur. I found that stress greatly reduced most C cycling indicators and inhibited nitrification, but tended to increase N mineralization, particularly as the soils adapted to stress. Legume residues added to the stressed soils stimulated catabolic processes (i.e. respiration and net N mineralization) to a much greater extent than anabolic processes (accumulation of biomass C), suggesting a reduced efficiency in the stress-adapted community. Compost addition did not affect how C mineralization or microbial C responded to stress. However, it consistently increased N mineralization, particularly when legume residues were added to the stressed soils, suggesting compost facilitated a community shift towards one with a relatively low need for N relative to C. In the third and fourth parts of this work, I explored the potential effects of long-term compost and cover crop incorporation on soil health by doing a full assessment of biological, physical, and chemical soil health indicators in a long-term field research experiment in which corn and processing tomato have been either conventionally or organically farmed for over 25 years. In this two-pronged approach, I firstly characterized the sensitivity of the different soil health indicators to management, as well as how they vary with growing season, crop type, and sampling date. In the second part, I assessed the management effects on three essential soil functions: C storage, N mineralization and crop yields. I assessed which indicators are most closely and consistently related to these functions across years and crop types. Finally, I used structural equation modeling to test the hypothesis that healthy soils lead to less stressed crops and thus to higher yields. This study found that organic inputs had strongly increased organic matter and biological activity at these sites. Indicators of C storage and especially biological activity tended to fluctuate between years and sampling dates, but were generally unaffected by crop phase. Different indicators were appropriate for predicting C storage and N mineralization, and I concluded that a holistic soil health assessment should include both indicator types. However, since yields were lower in the organic than conventional system for both crops, my hypothesis that healthier soils would produce healthier crops was not supported. The results of the path analysis suggested the presence of an unmeasured factor strongly related to both management and yields. In the years of this study this factor was likely disease pressures which built up over the years in the organic system. Together, the data from these experiments will be useful to growers, researchers, and policymakers wishing to assess the potential benefits, limitations and risks of different organic amendments over short and long time scales.

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