Nematode indicators as integrative measures of soil condition in organic cropping systems

Even though indicators based on nematode community composition are among the best developed metrics of soil health, little research has been done to support their application to management. This work tested the hypothesis that organic farming systems could enhance nematode community structure where soil resource condition had been enhanced by reducing disturbance and using organic fertility inputs. We tested this hypothesis by exploring the relationship between soil biology and the condition of the soil resource in an organic trial using the Nematode Maturity (MI), Plant Parasitic (PPI), Enrichment (EI), Channel (CI), and Structure (SI) indices. Shifts in nematode indices occurring during the growing season were compared with measures of labile soil organic matter and N availability [particulate organic matter-C and -N (POM-C, POM-N), particulate organic matter C:N ratio (POM C:N), potentially mineralizable N (PMN), hydrolysable N estimated with the Illinois-N test (IL-N), and heterotrophic activity estimated with fluorescein diacetate hydrolysis (FDA)]. Soil samples were collected four times during a single growing season from plots that had been transitioned from conventional to organic management under Ley-, Row Crop- and Vegetable-based regimes. Each of the three transition scenarios included three subplots that were either supplied with organic matter from raw and composted dairy manure or cover crop residues. High EI and low CI values revealed a greater dominance of the bacterial decomposition pathway in soils transitioned under the Ley system and values were positively associated with measures of microbial activity and available N. Increases in the PPI values were stimulated where nutrient availability (and likely crop root growth) were enhanced. The SI was positively associated with the POM fraction and declined after primary tillage. This signalled a shift in the community towards an early stage of succession and demonstrates the detrimental effects of tillage on soil food web complexity. Recommendations for soil stewardship that are guided by nematode food web indices would encourage practices that enhance soil organic matter, reduce the frequency of cultivation, and conserve structure.