O2 uptake, C metabolism and denitrification associated with manure hot-spots

O2, C and N metabolism in organic hot-spots (sites where the intensity of microbial respiration creates a high O2 demand) was studied with fresh or anaerobically digested liquid cattle manure as substrates. A gel-stabilized mixture of soil and manure, 16 mm thick, was sandwiched between layers of soil with a water content adjusted to field capacity, and incubated at 15°C for up to 3 wk. When fresh manure was used, O2 microprofiles demonstrated an O2 penetration into the hot-spot of < 1 mm after 1–3 d, increasing to ca. 2 mm after 3 wk. During this time, O2 uptake rates decreased from 100–150 to ca. 50 nmol O2 cm−2 h−1. With digested manure, the lower C availability in this substrate resulted in O2 penetration depths of 3–4 mm and O2 uptake rates of <30 nmol O2 cm−2 h−1 throughout the 3 wk. Maximum denitrification rates were also consistently lower with digested manure (4 nmol N cm−2 h−1) than with fresh manure (18 nmol N cm−2 h−1). A numerical model of NO3− transport indicated that denitrification was limited by the availability of NO3− during the first week in the fresh manure treatment, and that the soil was the only significant source of NO3− during at least 3 d; after this time nitrification at the soil-manure interface became increasingly important. After the first week with fresh manure, and throughout the experiment with digested manure, C availability apparently regulated denitrification.