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

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.