Incorporation of Nitrogen‐15‐Labeled Amendments into Physically Separated Soil Organic Matter Fractions

Abstract

Physically separated soil organic matter (SOM) fractions may take different functions in soil N dynamics. We studied the effect of long‐term organic matter (OM) management and different soil biological activity on the incorporation of N added with organic and mineral amendments into aggregate fractions and size density fractions. We applied 15N‐labeled sheep feces, urine, and mineral fertilizer to microplots installed in plots of conventional (CONMIN) and bio‐organic (BIOORG) cropping systems. Soil sampled 112 d after amendment was separated into macro‐, microaggregates, and microstructures. Aggregates were then fractionated into free light fraction (LF), intra‐aggregate particulate organic matter (iPOM), and the mineral‐associated organic matter fraction (MF). Of total soil N, 67% was contained in macroaggregates. Size density fractionation of aggregates revealed that about 60% of soil N was stored in MF while LF and iPOM contained together <3% of soil N. Despite long‐term OM input and higher soil biological activity in BIOORG than CONMIN the two soils did not differ in the distribution and content of N in aggregate and size density fractions. Recovery of 15N in nonfractionated soil ranged from 20% (SlurryF) to 25% (SlurryU) of originally applied 15N. The small macroaggregates were for each amendment the major sink (7–12% of applied 15N). In all aggregates and for all amendments, MF was the most important 15N sink, totally containing between 6.6% (SlurryF) to 11.6% (SlurryU) of applied 15N. Less than 1% of applied 15N was recovered in LF, and even less (<0.5%) in iPOM. The proportion of amendment‐derived N in aggregate fractions and in several size density fractions (LF, fine iPOM, MF) was higher for urine than for feces and mineral fertilizer. Recovery of urine‐derived 15N was greater in aggregate fractions of BIOORG than CONMIN soil. During dispersion of aggregates to obtain iPOM and MF, about 27% of total soil N and between 37 and 55% of 15N contained in non‐fractionated soil was lost, showing the importance of aggregation to protect N.