Soil Enzyme Activities, Microbial Communities and Carbon and Nitrogen Availability in Organic Agroecosystems Across an Intensively-Managed Agricultural Landscape

Authors: BOWLES, TIMOTHY - University Of California; Acosta-Martinez, Veronica; Calderon, Francisco; JACKSON, LOUISE - University Of California

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2013
Publication Date: 1/1/2014
Citation: Bowles, T.M., Acosta Martinez, V., Calderon, F.J., Jackson, L.E. 2014. Soil enzyme activities, microbial communities and carbon and nitrogen availability in organic agroecosystems across an intensively-managed agricultural landscape. Soil Biology and Biochemistry. 68:252-262.

Interpretive Summary: Soil microbes provide services that are crucial for sustainable agricultural systems, such as liberating nutrients from organic matter to make them available for plants. Better management of soil microbes requires knowledge of how the microbial community changes across working farms using different management practices and on soils of differing quality. This study focused on how soil microbes differed across a set of 13 organically-managed fields growing Roma-type tomatoes in the Central Valley of California. These fields used different types of organic amendments (manure, compost, and vetch cover crops) and had a three-fold range of soil organic carbon (C), a crucial indicator of soil quality. Other soil characteristics, including texture and pH, were similar. The composition of the microbial community changed depending on the type of organic amendment used, but overall was relatively similar across the fields. This similarity may reflect the long history of intensive agriculture in this area and the high rates of disturbances, such as tillage, that occur even in organic systems. Markers of arbuscular mycorrhizal fungi, an important plant symbiont, did decrease with increasing P availability, which can often build up with long-term manure application. In contrast, enzyme activities involved in nutrient cycling were highly differentiated across the fields. For instance, fields that used composted green waste and had more soil C tended to have greater enzyme activities of N cycling, while fields that used manure and had higher inorganic N tended to have greater enzyme activities of C cycling. Our results suggest that current and historical agroecosystem-management has effectively manipulated soil microbial functions to favor the enhancement of nitrogen availability, nitrogen retention, and C stabilization in these fields.

Technical Abstract: Variability in the activity and composition of soil microbial communities may have important implications for the suite of microbially-derived ecosystem functions upon which agricultural systems rely, particularly organic agriculture. An on-farm approach was used to investigate microbial communities and soil carbon (C) and nitrogen (N) availability on 13 organically managed fields growing Roma-type tomatoes, but differing in nutrient management, across an intensively-managed agricultural landscape in the Central Valley of California. Soil physicochemical characteristics, potential activities of nine soil enzymes involved in C, N, phosphorus (P), and sulfur (S) cycling, and fatty acid methyl esters (FAMEs) were measured during the growing season and evaluated with multivariate approaches. Soil texture and pH in the 0-15 cm surface layer were similar across the 13 fields, but there was a three-fold range of soil C and N as well as substantial variation in inorganic N and available P that reflected current and historical management practices. Redundancy analysis showed distinct profiles of enzyme activities across the fields, such that C-cycling enzyme potential activities increased with inorganic N availability while those of N-cycling enzymes increased with C availability. Differentiation of microbial community composition by organic amendments (manure vs. composted green waste vs. legume crops) was greater than among individual fields, but overall, FAMEs suggest relatively minor variation in microbial community composition, at least at the level of broad taxonomic indicators, likely reflecting the high disturbance and low complexity in this landscape. Variation in potential enzyme activities was better accounted for with soil physicochemical characteristics than microbial community composition, suggesting high plasticity of the resident microbial community to environmental conditions. These patterns suggest that current and historical agroecosystem management has effectively manipulated soil microbial functions to favor the enhancement of N availability, N retention, and C stabilization in the various types of organic agroecosystems across the landscape. The on-farm approach provided a wide range of farming practices and soil characteristics to reveal how microbial-derived ecosystem functions can be effectively manipulated to enhance nutrient cycling capacity.