Crop plant contributions to root zone available carbon and microbial transformations of nitrogen

Available C produced by crop roots during growth can profoundly influence microbial activities that regulate N cycling in soil which are important to N-use efficiency and quality of air and water environments. The objectives of this research were to establish a quantitative method for determining root-derived available C, and evaluate its effects on microbial transformations of N using a $\rm \sp{15}NH\sb4\sp{15}NO\sb3$ fertilizer tracer. We hypothesized that increased release of root-derived C during early crop growth would stimulate microbial denitrification and immobilization and result in a loss of plant available scNO$\sb3\sp-$-N. In a greenhouse study, we used $\sp{13}$C abundance change in soil microbial biomass to quantify contributions of corn (Zea mays L.), a C$\sb4$ plant, to root zone available C during growth in soil with a long history of C$\sb3$ vegetation. Using this approach, we estimated that root released C accounted for 12% (210 kg C ha$\sp{-1}$) of measured C fixed by corn during early growth (4 weeks) and 5% at maturity when root released C totaled 1135 kg C ha$\sp{-1}$. Average daily production of root-derived available C was greatest during 4 to 8 weeks (7 kg C ha$\rm\sp{-1}\ d\sp{-1}$) while 4 to 11% of the soil microbial biomass came from these C sources; root-derived biomass was maximum at corn maturity (15%) which represented 402 kg ha$\sp{-1}$ of root-derived available C. Of the $\sp{15}$N remaining in the bare and cropped soils, an average of 23 and 16% was found in microbial biomass, 64 and 2% was in mineral $\sp{15}$N form, and 13 and 82% as non-biomass organic N, respectively, suggesting that N cycling was enhanced by root-derived C. Denitrification and N$\sb2$O losses from the planted soil ranged from 0.02 to 3.4 kg N ha$\rm\sp{-1}d\sp{-1}$ with N$\sb2$ comprising 70-99% of denitrification N gases when soils were wetted to 85-95% water-filled pore space (WFPS). Maximum denitrification rate was 1.5 times greater and cumulative denitrification losses 77% greater during early growth stages in planted soil as compared to bare soil when adequate nitrate ($\u3e$2-3 mg kg$\sp{-1}$) was present in soil. Presence of plants increased denitrification losses from soil by 29% during early growth stages when release of root-derived C was greatest