Abstract
<p>Organic field crop systems are characterized by complex rotations with high spatial and temporal vegetative diversity, an enhanced use of legumes, and reduced external nutrient (nitrogen (N) and phosphorus (P)) use. At the same time, a core premise of certified organic agriculture is that this farming system provides benefits to soil health via enhanced microbial diversity. The following short review, drawing primarily upon selected studies from North America, examines the impact of farming systems, and various management strategies within these, on soil organic matter, N and P dynamics, and soil microbial and macrofaunal abundance and diversity. Organic cropping systems are shown to provide benefits with respect to reduced farm N and P surpluses, in combination with maintenance of soil organic matter and improved soil health. However, soil health benefits appear consistently achieved only for larger soil organisms partly due to the resilience of the soil microbial community. Recent research examining soil P dynamics and P uptake in relation to legume biological N<sub>2</sub> fixation and bacterial and mycorrhizal community diversity provide evidence of the resilience of the soil microbial community with respect to functionality, if not diversity of microbial community composition. These latter results may challenge organic agriculture core premises of consistent benefits to soil health via enhanced microbial diversity, but in its place may lead to an improved understanding of how specific cropping practices and production system intensity overall, rather than farming system per se, influences both nutrient cycling and soil ecosystem functioning.</p>
Highlights
Recent research examining soil P dynamics and P uptake in relation to legume biological N2 fixation and bacterial and mycorrhizal community diversity provide evidence of the resilience of the soil microbial community with respect to functionality, if not diversity of microbial community composition. These latter results may challenge organic agriculture core premises of consistent benefits to soil health via enhanced microbial diversity, but in its place may lead to an improved understanding of how specific cropping practices and production system intensity overall, rather than farming system per se, influences both nutrient cycling and soil ecosystem functioning
A core premise of certified organic agriculture is that this farming system benefits soil health via enhanced microbial diversity, with ancillary benefits with respect to soil quality, nutrient dynamics and broad farm ecology generally (Stockdale & Watson, 2009; Gomiero et al, 2011)
What do we mean by soil health? Faced with growing evidence of the remarkable resilience especially of the soil microbial community, – is soil microbial diversity per se a key goal or is it more appropriate and beneficial to focus on the abundance and diversity of higher trophic levels? Postma-Blaauw et al (2010) for example, found the abundance of taxonomic groups such as earthworms, microarthropods and nematodes was much more affected by agricultural intensification than that of bacteria, fungi and protozoa
Summary
A core premise of certified organic agriculture is that this farming system benefits soil health via enhanced microbial diversity, with ancillary benefits with respect to soil quality, nutrient dynamics and broad farm ecology generally (Stockdale & Watson, 2009; Gomiero et al, 2011). The following short review is designed to offer reflection, and hopefully generate a discussion, on these important questions This may challenge the organic agriculture premise of consistent benefits to soil health via enhanced microbial diversity, but in its place lead to an improved understanding of how soil microbial resilience, and production system intensity and specific cropping practices, rather than farming system per se, influence these important soil system outcomes. Such an improved understanding of plant and soil biota relationships as influenced by farming practices will provide a more refined framework for approaches to beneficial management of these relationships and their influence on nutrient dynamics and efficiency and farming system productivity. Vol 4, No 3; 2015 primary, but not exclusive, focus on organic cropping systems in North America, and with respect to nutrients, a particular focus on nitrogen (N) and phosphorus (P) dynamics in relation to soil health
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