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 C-13 abundance of soil microbial biomass was used to quantify contributions from maize (Zea mays L.), a C-4 plant, to root zone-available C during growth in soil with a long history of C-3 vegetation. Effects of root-derived available C on microbial transformations of N were also evaluated using a (NH)-N-15:5NO(3) fertilizer, tracer. Root-released C (microbial respired C-4-C + soil residue C-4-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 C-4-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 C-4 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 N-15 remaining in bare and cropped soils, averages of 23 and 16% (IO 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 N-15 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 N-2 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)) 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. Published by Elsevier Science Ltd.