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Soil Biology & Biochemistry

Prediction of Gross and Net Nitrogen Mineralization-Immobilization-Turnover from Respiration

^a Royal Veterinary and Agricultural Univ., Plant and Soil Sci. Lab., Thorvaldsensvej 40, 3. sal, 1871 Frederiksberg C, Denmark
^b Division of Precision Agriculture, Swedish University of Agricultural Sciences, P.O. Box 234, SE-532 23 Skara, Sweden
^c Department of Horticulture and Crop Science, Agricultural University of Norway, P.O. Box 5022, N-1432 Aas, Norway

^* Corresponding author (jelu{at}kvl.dk)

Prediction of net N mineralization is required for optimization of the synchronization of N supply with plant N demand. Net N mineralization is the outcome of two concurrent and oppositely directed processes: gross N mineralization and gross N immobilization turnover (MIT). Consequently, an improved understanding of MIT can potentially improve our capability to predict net N mineralization patterns. The aim of the study was to measure MIT and respiration rates of widely differing plant materials and look for relations between them. Eight plant residues with a very wide range in C to N ratios were incorporated into soil and incubated at 16°C. During 2-d intervals (5–6, 25–26, and 55–56 d after incorporation), MIT and respiration rates were determined. The respiration and gross N immobilization rates were correlated (R² = 0.74), whereas respiration and gross N mineralization rates were less well correlated (R² = 0.41). The correlation was improved (R² = 0.89) when only the data from the first incubation period and the C/N ratio of acid detergent solubles (ADS) were taken into account. Assuming that the soil microorganisms have a C use efficiency of 50%, this study showed that the gross N mineralization rate made up only 30% of the total gross litter N decomposition rate (i.e., the remaining 70% being directly assimilated by soil microorganisms in organic form). Net N mineralization rates, derived from the difference between predicted rates of gross N mineralization and gross N immobilization, could explain up to 64% of the variability in measured net N mineralization rates. In conclusion, this study revealed that MIT in the initial phase of decomposition can be derived from data on C mineralization and the C/N ratio of ADS, which can simplify the process of calibrating and validating mechanistic models and thereby improve our capability of predicting net N mineralization.

Abbreviations: ADS, acid detergent solubles • DW, dry weight • MIT, N immobilization turnover • NDS, neutral detergent-solubles • WS, hot-water solubles

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