Long-term manure application to improve soil macroaggregates and plant-available nitrogen in a Mollisol

Abstract

Long-term incorporation of manure has shown many benefits to improve soil productivity, but its effects on nitrogen (N) partitioning in soil aggregates and availability to plants is not well understood. The aim of this study was to examine the influence of different long-term field fertilization regimes on N partitioning and availability in soil aggregates. Soils from a field after 25-yr treatments of no fertilization (CK), mineral fertilizer (NPK), NPK with stover return (NPKS), and NPK with manure application (NPKM) were examined for N partitioning after adding 15N-labelled urea and incubating for 42 days. Using a wet sieving procedure, soil was separated into macroaggregates (> 250 μm, coarse particulate organic matter or cPOM), microaggregates (53–250 μm), and silt + clay (< 53 μm, inter mineral-associated OM or inter-MOM). The OM in microaggregates was further separated into fine free POM (ffPOM), physically protected intra-aggregate POM (iPOM), and intra mineral-associated OM (intra-MOM). Total N and 15N concentrations were determined. All fractions except ffPOM (limited mass) were used to cultivate ryegrass (Lolium perenne L.) and 15N uptake was determined. Mass of macroaggregate was 10 % higher from NPKM and NPKS with significantly higher use efficiency than that from NPK while the opposite was true for intra-MOM fraction with no differences in other fractions. The 15N retained in cPOM, intra-MOM, and inter-MOM ranged from 24 to 34%, 28–37 %, and 33–38 %, respectively, and that from ffPOM and iPOM was 1–4% of all retained in aggregates. Ryegrass N uptake was in the range of 14–19 %, 14–24 %, and 3–5% for the cPOM, inter-MOM, and the intra-MOM, respectively. However, the total amount of 15N fertilizer retained in aggregates from the NPKM treatment (14 %) was the lowest due to its higher amount (86 % of 15N) as in soluble phase compared to other treatments (78–83 %). This research concludes that long-term manure application resulted in increased formation of macroaggregates, higher plant-available N in the aggregates, and larger amount of soluble N that contributed to the high yield in the field illustrating the role of manure to improve soil productivity and sustain crop production.