Soil quality, microbial community structure, and organic nitrogen uptake in organic and conventional farming systems

Soil quality was investigated over two years in adjacent, commercial organic and conventional strawberry fields near Watsonville, California. The 13 organic and conventional fields pairs were matched for strawberry variety, soil type, and other environmental conditions. Organically managed fields were found to contain greater concentrations of organic carbon, nitrogen, zinc, boron, microbial parameters and enzyme activities. Differences in microbial community structure and functional diversity were studied using DNA functional gene microarrays. Organic management resulted in significantly higher abundance and diversity of detected genes with management exerting a stronger influence than soil type. Significant correlations were found between overall slide signal intensity (SI) and microbial biomass and between cellulase gene SI and cellulase activity. These results demonstrate the potential of high throughput functional gene arrays to detect ecologically relevant changes in microbial communities in response to management and soil type.;Organic N uptake in three modern, three pre 1940 and perennial wheat varieties was also studied. Glycine uptake increased with increasing soil concentration. Partial soil sterilization or pretreatment with differing levels of nutrients resulted in no significant differences in glycine uptake. Glycine-N constituted between 3.9 and 8.1 percent of total N uptake with perennial wheat utilizing significantly more organic N than annual varieties and modern annual varieties significantly more than old varieties. These results could have significance in the development of wheat adapted for organic and low-input systems. Preliminary work with strawberries suggests that wild strawberry species utilize up to 10 percent of their total N as amino acid and that cultivated strawberries utilize only negligible amounts. More work is needed with more varieties to confirm this result.;In order to determine if more organic N was available for plant uptake in organic systems, amino acid N available in soil solution and native and potential soil protease activity were measured in organic and conventional strawberry fields. While no significant differences were found, amino acid turnover was shown to be slower in fine textured soils with a similar trend in organically managed soils, suggesting less microbial competition for amino acid N under these higher carbon conditions.