Effects of long-term crop management on nematode trophic levels other than plant feeders disappear after 1 year of disruptive soil management

Nematode community analysis may provide a useful tool to quantify soil health. Nematode communities were monitored for 5 years during a 12-year period in the sustainable agriculture farming systems (SAFS) project at UC Davis, where conventional (CONV), low-input (LOW) and organic (ORG) management treatments were compared. After the completion of three 4-year crop rotation cycles, a uniform crop of oats was grown in 2001. The composition of the nematode genera was different from year to year, but there were significant management effects on genus composition in each year, with the CONV treatment being significantly different from the LOW and ORG treatments. Important contributors to the differences in genus composition among treatments were plant parasitic nematodes. Nematode community indices (enrichment (EI), basal (BI) and channel (CI) indices) of the CONV treatment differed from those of the ORG and LOW treatments in 1993, 1994, 1995 and 2000, but not in 2001. The difference in structure index (SI) among management treatments was significant in 1995 and 2000. EI and SI were generally lower, and BI and CI higher in CONV than in LOW and ORG treatments. There were significant crop effects on the community indices throughout the years. Even in 2001, there was a residual effect of the crop grown in 2000 on most nematode community indices. Differences in EI, BI and CI among crops were consistent, while those in SI were not. Meloidogyne javanica (Treub) Chitwood, juveniles added to various soil samples were reduced by 68% in soil where nematode trapping fungi had been added and which had low BI (12) and low CI (20) values. Soil from SAFS plots with a high BI (47) and high CI (70) after 1 year of oats and ploughing, suppressed root knot juveniles much less. There were significant negative correlations between BI and root knot nematode (RKN) suppression (−0.72) and between CI and RKN suppression (−0.74). Thus, BI and CI appeared to be most valuable as indicators for long-term effects of management on nematode suppression. However, BI and SI may be more suitable as general indicators for the health status of a soil, since CI can be high in highly disturbed agro-ecosystems as well as in undisturbed natural ecosystems. A high BI would indicate poor ecosystem health, while a high SI would indicate a well-regulated, healthy ecosystem. For agricultural soils the presence of large populations of plant parasitic nematodes forms an additional indication of poor ecosystem health, as natural regulation is limited in this case.