Soil food webs and carbon dynamics in response to conservation tillage in California

Reducing disturbance by tillage and addition of crop residues affects soil biota and their role in soil C storage. For 1 yr in a field station trial in Davis, CA, these treatments were compared: no-tillage + continuous cropping, no-tillage + fallow, standard tillage + continuous cropping, and standard tillage + fallow. The continuous cropping treatment consisted of tomato (Lycopersicon esculentum Mill.)/sorghum-sudangrass [Sorghum bicolor (L.) Moench]/garbanzo (Cicer arietinum L.)/cowpea cover crop [Vigna unguiculata (L.) Walpers ssp. unguiculata]. The fallow rotation omitted the sorghum-sudangrass and cowpea cover crops. No-tillage + continuous cropping resulted in significant changes in the surface layer (0-5 cm): higher microbial biomass C, more fungi as indicated by ergosterol and phospholipid fatty acid analysis, and higher soil NO₃^- in summer, and higher pH, soluble K⁺, and Olsen P at the end of the experiment. At lower depths (5-15 and 15-30 cm), few differences were observed. Total soil C (at 0-30 cm) was least with standard tillage + fallow, the typical management practice in the region. The soil food web, as indicated by the nematodes, did not become more complex with no-tillage + continuous cropping, contrary to expectations, possibly because higher trophic level nematodes had been eliminated after decades of cultivation. The bacterial decomposition pathway dominated the surface layer in all treatments, but, with no-tillage, opportunistic (colonizer-persistent Group 1) bacterial feeders greatly decreased with depth. Plant productivity, except for weeds, was reduced by no-tillage, especially in the garbanzo crop. By decreasing disturbance and increasing fungi, no-tillage + continuous cropping appears to have accelerated soil C storage but management alterations are needed to produce higher crop biomass in this Mediterranean-type climate.