Effect of Water‐Filled Pore Space on Carbon Dioxide and Nitrous Oxide Production in Tilled and Nontilled Soils

Abstract

Abstract The percentage of soil pore space filled with water (percent water‐filled pores, % WFP), as determined by water content and total porosity, appears to be closely related to soil microbial activity under different tillage regimes. Soil incubated in the laboratory at 60% WFP supported maximum aerobic microbial activity as determined by CO 2 production and O 2 uptake. In the field, % WFP of surface no‐tillage soils (0–75 mm) at four U.S. locations averaged 62% at time of sampling, whereas that for plowed soils was 44%. This difference in % WFP was reflected in 3.4 and 9.4 times greater CO 2 and N 2 O production, respectively, from surface no‐tillage soils over a 24‐h period as compared to plowed soils. At a depth of 75 to 150 mm, % WFP values increased in both no‐tillage and plowed soils, averaging approximately 70% for no tillage compared with 50 to 60% for plowed soils. Production of CO 2 in the plowed soils was enhanced by the increased % WFP, resulting in little or no difference in CO 2 production between tillage treatments. Nitrous oxide production, however, remained greater under no‐tillage conditions. Substantially greater amounts of N 2 O were produced from the N‐fertilized soils, regardless of tillage practice. Production of CO 2 and N 2 O was primarily related to the % WFP of tillage treatments although, in several instances, soil‐water‐soluble C and NO ‐ 3 levels were important as well. Calculations of relative aerobic microbial activity between no‐tillage and plowed soils, based on differences in % WFP relative to maximum activity at 60%, indicated linear relationships for CO 2 and N 2 O production between WFP values of 30 to 70%. Below 60% WFP, water limits microbial activity, but above 60%, aerobic microbial activity decreases—apparently the result of reduced aeration.