Measuring and modelling seasonal variation of gross nitrification rates in response to long-term fertilisation

Abstract. The formation of nitrate (nitrification) in soils is an important process that influences N availability for plant uptake and potential N losses as well. Gross nitrification is an effective measure by which to test mechanistic ecosystem models for predictability because gross rates can widely differ between sites, even if net production is similar between these sites.

A field experiment was designed to (i) determine gross nitrification rates in response to fertilisation and (ii) to verify the idea that seasonal variations of gross rates in soils can be readily predicted by soil moisture and soil temperature.

Gross nitrification rates were measured by a Barometric Process Separation (BaPS). The BaPS measurements were validated with the commonly used ¹⁵N pool dilution technique measurements at six times. In general, the rates determined from both measurement approaches were in the same order of magnitude and showed a good correlation.

The effects of 100 years of fertilisation (mineral fertiliser, manure and control) on gross nitrification rates were investigated. During 2004 soil samples from the long-term "static fertilisation experiment" at Bad Lauchstädt were sampled weekly and were measured in the laboratory under field conditions and subsequently under standardised conditions (16°C soil temperature and −30 kPa matrix potential) with the BaPS system. Gross nitrification rates determined under standardised conditions did not show any seasonal trend but did, however, reveal a high temporal variability. Gross nitrification rates determined by the BaPS-method under field conditions showed also a high temporal variability and ranged from 5 to 77 μg N h⁻¹ kg⁻¹ dry mass, 2 to 74 μg N h⁻¹ kg⁻¹ dry mass and 0 to 49 μg N h⁻¹ kg⁻¹ dry mass with respect to manure, mineral fertiliser, and control. The annual average was 0.34, 0.27 and 0.19 g N a⁻¹ kg⁻¹ dry mass for the manure site, mineral fertiliser site and control site, respectively. On all sites gross nitrification revealed a strong seasonal dynamic. Three different models were applied for reproducing the measured results. Test models could explain 75% to 78% of variability at the manure site, 66% to 77% of variability at the mineral fertiliser site, and 39% to 63% of variability at the control site. The model parameterisation shows that the temperature sensitivity of gross nitrification differs between the three neighbouring sites. Hence, a temperature response function in an ecosystem model has to consider the site specificity in order to adequately predict the effects of future climate change on the soil N cycle.