Nitrogen availability after repeated additions of raw and anaerobically digested 15N-labelled pig slurry

Quantifying short- and long-term availability of animal manure nitrogen (N) is of practical interest to ensure adequate crop yield, minimize N pollution and reduce inputs of mineral-N fertilizer. We measured short-term carbon (C) and N dynamics after one or six repeated additions to soil (every 56 days) of ammonium sulphate (AS) and undigested (PS) and anaerobically digested (DPS) pig slurry in a laboratory incubation experiment. Soil CO2 emissions, pH and mineral N (ammonium+nitrate) were measured during the period of 0–56 days following fertilizer additions. An accompanying experiment was conducted with similar but 15N-labelled fertilizers to measure soil mineral N 56 and 112 days after one, three or six repeated additions, and to estimate the increase in slurry available N after repeated additions. Nitrogen from slurries potentially available for plants (77–91, 44–58 and 57–66% of added N for AS, PS and DPS, respectively) was close to that supplied in mineral form with the fertilizer, indicating negligible net mineralization of slurry organic N, after both one and repeated additions. In fact, soil mineral N increased in most of the treatments because of repeated additions, but the increases were small, on average 0.5–2.1% of added N per period of 56 and 112 days. Calculations of availability of fertilizer N based on unlabelled N were equally precise compared with those estimated with 15N, but trends over time were more variable. We conclude that many repeated additions (simulating a long manuring history) are needed to obtain a marked increase in available slurry N, even under controlled conditions.