Gross nitrogen mineralisation rates in pastoral soils and their relationships with forms of soil organic nitrogen and microbial and enzymatic activities

Abstract

The rate of nitrogen (N) mineralisation is an important parameter that determines the amount of soil-N supply to plants and indicates the potential impact of N on the wider environment. A better estimation of N mineralisation rate from soil organic matter is required to improve fertiliser recommendations and reduce environmental impacts. In this study, the rate of N mineralisation and its relationship with soil organic N, microbial biomass, enzymatic activity and environmental conditions was determined in a range of soils under glasshouse, incubation, and field conditions.

In the glasshouse experiment, six soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand were studied. The soils were kept under controlled moisture (-10 to -30 kPa) and temperature (15 to 20°C) conditions. The gross N mineralisation rate ranged from 0.76-5.87 µg N g⁻¹ soil day⁻¹ in the six soils. Nearly all the parameters measured, for example, amino acid-N (AA-N), amino sugar-N (AS-N), microbial biomass carbon (MB-C), microbial biomass-N (MB-N) and protease activity were greater in the Fleming and Kairanga soils. This was probably due to these two soils being more fertile with higher organic C and N contents compared with the other soils. The gross N mineralisation rate was positively correlated to the amount of AA-N, ammonia-N (NH₃-N), total hydrolysable-N (TH-N), MB-C, MB-N, protease activity, organic C and N. Stepwise regression analysis showed that the gross N mineralisation rate was defined by the concentration of MB-C, AA-N and TH-N:

Gross N mineralisation rate (µg N g⁻¹ soil day⁻¹) = -0.566 + 0.001(MB-C µg C g⁻¹ soil) + 0.002(AA-N µg N g⁻¹ soil) + 0.003 (TH-N µg N g⁻¹ soil).

In the incubation and field studies, the soils were treated with farm dairy effluent (FDE) or urea fertiliser at the rate of 200 kg N ha⁻¹ yr⁻¹. Compared with the control, the gross N mineralisation rate was greater in the FDE treated soils in both the incubation study (0.66-5.85 µg N g⁻¹ soil day⁻¹) and in the field study (1.86-6.84 µg N g⁻¹ soil day⁻¹). Application of FDE also increased MB-C, MB-N, protease activity and AA-N in both studies. Urea application did not have a significant effect on the gross N mineralisation rate, MB-C, MB-N, protease activity. In the incubation study, the stepwise regression model showed that the gross N mineralisation rate was determined by the concentration of MB-C and AA-N:

Gross N mineralisation rate (µg N g⁻¹ soil day⁻¹) = 12.315 + 0.004(MB-C µg C g⁻¹ soil) + 0.11(AA-N µg C g⁻¹ soil).

In the field study, the gross N mineralisation was described by the concentration of MB-N alone:

Gross N mineralisation rate (µg N g⁻¹ soil day⁻¹) = 0.256 + 0.026(MB-N µg g⁻¹ soil).

These studies confirm that N mineralisation rate is affected by the mineralisable forms of soil organic N pools and microbial biomass. However, these variables were only able to account for up to 40 % of the variations observed in the gross N mineralisation rate in the glasshouse and incubation studies, and only 30 % in the field study. Clearly there are other variables and factors that affect N mineralisation that still need to be examined and quantified.

Several 'simple quick test' methods were evaluated for their ability to predict the N mineralisation potential of pastoral soils. It was found that autoclave extracted-NH₄⁺, 0.025 M HCl extracted-NH₄⁺ and 0.5 M NaHCO₃ methods were good indicators of N mineralisation rate. The equations derived and the coefficients are as follows:

i) Autoclave extracted-NH₄⁺ (R₂ = 0.700; P<0.001) Gross N mineralisation rate (pg N g⁻¹ soil day⁻¹) = -0.81 + 7.41(1- e⁻⁰•⁰⁰⁷ˣ) x= NH₄ + (µg g⁻¹ soil) ii) 0.025 M HCl extracted-NH₄⁺ (R₂ = 0.829; P<0.001) Gross N mineralisation rate (µg N g⁻¹ soil day⁻¹) = 1.08 + 3.831(1- e⁻⁰•⁰²ˣ) x= NH₄+ (µg g⁻¹ soil)

iii) 0.5 M NaHCO₃ extracted-NH₄⁺ (R₂ = 0.806; P<0.001) Gross N mineralisation rate (µg N g⁻¹ soil day⁻¹) = 0.72 + 4.041(1- e⁻⁰•⁰³ˣ) x= NH₄⁺ (µg g⁻¹ soil)