Characterizing bioactive phosphorus of soil aggregates in conventional and manure-based cropping systems

摘要

In areas of intensive animal agriculture, applications of manure often result in accumulation of inorganic and organic phosphorus (P) in the near-surface zone. Organic fanning systems depend heavily upon organic sources of nutrients, largely from animal manures. A long-term field study was conducted to compare the potential losses of sediment-associated nutrients from no-till (NT), conventional till (CT), and organically managed system (ORG) that received repeated poultry manure applications. After a decade of continuous organic and conventional conservation management, all soils showed nutrient levels that were characteristic of P-enriched soils in the surface 0 to 5-cm depth. Concentrations in combination with the aggregate size distribution showed that about 70% of soil P was concentrated in macroaggregates ranging from 0.2 to > 2 mm in size in the NT treatment whereas, a greater percentage of soil P was found in microaggregates of < 0.2 mm in CT and ORG treatments. The high intensity and frequency of tillage in CT and ORG treatments negated the benefits of the high carbon loading from cover crops and poultry manure applications, resulting in soil conditions that were less conducive to the formation and/or stability of large macroaggregates than those under NT management of these Ultisols. Nutrient management practices for erosion-prone manure-amended lands should focus on both inorganic- and organic bioactive P release from soils as both are potential contributors to water quality degradation from eroded sediments and dissolved P.