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

Root-derived C influences soil microbial activities that regulate N transformations and cycling in soil. The change in 13C abundance of soil microbial biomass was used to quantify contributions from maize (Zea mays L.), a C4 plant, to root zone-available C during growth in soil with a long history of C3 vegetation. Effects of root-derived available C on microbial transformations of N were also evaluated using a 15NH415NO3 fertilizer tracer. Root-released C (microbial respired C4C + soil residue C4C) accounted for 12% (210 kg C ha−1) of measured C fixed by maize at 4 wk and 5% at maturity when root-released C totaled 1135 kg C ha−1. Of the C4C remaining in soil, only 18–23% was found in microbial biomass, indicating either a rapid turnover rate of biomass or a lower availability of C4 substrates. Average daily production of root-derived available C was greatest during 4–8 wk maize growth (7 kg C ha−1 d−1) when 4–11% of the soil microbial biomass came from this C source. At maize maturity, 15% of the microbial biomass (161 kg C ha−1) came from root-derived available C, which totaled 402 kg ha−1. Of the 15N remaining in bare and cropped soils, averages of 23 and 16% (10 and 2 kg N ha−1) were found in microbial biomass, and 64 and 2% (28 and 0.2 kg N ha−1) were in inorganic 15N form, leaving 13 and 82% (6 and 10 kg N ha−1) as non-biomass organic N, respectively; this suggests that N cycling through microbial biomass was enhanced by root-derived C. Denitrification and N2O losses from planted soils were low (1–136 g N ha−1 d−1) when soil water-filled pore space (WFPS) was <50%, but increased to 0.02–3.4 kg N ha−1 d−1 when soils were wetted to 85–95% WFPS when N2 comprised 70–99% of denitrification products. The maximum denitrification rate was 1.5 times greater, and the cumulative denitrification losses 77% greater during early growth stages in planted soil as compared to bare soil when adequate NO3−N (> 2–3 mg kg−1) was present in the soil. The presence of maize plants increased denitrification losses from soil by 19 to 57% (average of 29%) during early growth stages when the release of root-derived C was greatest.