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SOIL FERTILITY & PLANT NUTRITION

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

^a Institute of Plant Sciences, Group of Plant Nutrition, ETH Zurich Research Station Eschikon, Lindau Switzerland
^b Acroscope Reckenholz-Tänikon (ART), Reckenholz, Zurich, Switzerland
^c Research Institute of Organic Farming (FiBL), Frick, Switzerland
^d Agricultural Univ., Dep. of Agrochemistry and Soil Science, Plovdiv Bulgaria

^* Corresponding author (astrid.oberson{at}ipw.agrl.ethz.ch).

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.

Abbreviations: AF, aggregate fraction • Amd, amendment • BIOORG, bio-organic cropping system • CONMIN, conventional cropping system • CS, cropping system • DM, dry matter • HF, heavy fraction • iPOM, intra-aggregate particulate organic matter • LF, light fraction • MF, mineral-associated organic matter fraction • MineralN, mineral fertilizer N (¹⁵NH₄¹⁵NO₃) • Ndflc, N derived from the labeled component of the amendment, OM, organic matter • POM, particulate organic matter • SDF, size density fraction • SlurryF, sheep slurry (unlabeled urine + ¹⁵N-labeled feces) • SlurryU, sheep slurry (¹⁵N-labeled urine + unlabeled feces) • SOM, soil organic matter • 0N, unfertilized control