Gross nitrogen mineralisation rates in pastural soils and their relationships with organic nitrogen fractions, microbial biomass and protease activity under glasshouse conditions

Abstract

In this study, gross nitrogen (N) mineralisation rates were determined in six pasture soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand. The soils were kept under controlled soil water potential (−10 to −30 kPa) and temperature (12–20°C) conditions in a glasshouse. The gross N mineralisation rates ranged from 0.76 to 5.87 μg N g⁻¹ soil day⁻¹ in the six soils and were positively correlated with the amount of amino acid-N (AA-N), ammonia-N (NH₃-N), total hydrolysable-N (TH-N), microbial biomass-carbon (MB-C), microbial biomass-N (MB-N), protease activity and organic C and N. A stepwise regression was used to generate equations that could best describe gross N mineralisation rates. Microbial biomass-carbon and AA-N were included in the equation that best described the gross N mineralisation rate:

$$ \begin{aligned} {\text{Gross}}\,{\text{N}}\,{\text{mineralisation}}\,{\text{rate }}{\left( {\mu {\text{g}}\,{\text{N}}\,{\text{g}}^{{ - {\text{1}}}} \,{\text{soil day}}^{{ - {\text{1}}}} } \right)} & \\ = - 0.566 + 0.001{\left( {{\text{MB - C}}\,\mu {\text{g}}\,{\text{C}}\,{\text{g}}^{{ - {\text{1}}}} \,{\text{soil}}} \right)} + 0.002{\left( {{\text{AA - N}}\,\mu {\text{g}}\,{\text{N}}\,{\text{g}}^{{ - 1}} \,{\text{soil}}} \right)}{\text{.}} & \\ \end{aligned} $$

The total amounts of N mineralised over the 1-year period were equivalent to between 492 and 1,351 kg N ha⁻¹ year⁻¹. Assuming mineralisation continues at a steady state throughout the year, this represents between 12 and 26% of the total organic N mineralised per year in these pasture soils.