Soil Based N Tests: Accounting for Mineralised Soil N On Contrasting Irish Grassland Soils.

Current nitrogen (N) recommendations do not give credit for N released from the soil through N mineralisation (N_o), which can vary greatly for temperate grassland soils. Reductions in N input costs and losses of N to the environment could be achieved through better prediction of N_o in a soil. However, a reliable soil N testing procedure is needed. Many studies have emphasised the reliability of biological N tests (incubations) and have used these tests as standards for predicting N_o. However, these studies also note the time consuming nature of these tests (7-210 days) and the implications for routine soil analysis.

The objective of this study was to evaluate the ability of a range of rapid soil N tests to differentiate between soils and quantify N_o for a range of Irish grassland soils, by comparison with the standard 7 day anaerobic incubation method (AI-7).

A total of 37 soils were sampled (10cm depth) from sites representing a range of soils used for grassland production in Ireland. A range of physical, chemical and biological soil properties were determined through laboratory analysis for each soil. The biological standard AI-7 test was conducted to estimate N_o for all 37 soils. The rapid N tests evaluated included, (1) Cold and (2) Hot 2M KCl extractable nitrate (NO₃^--N), nitrite (NO₂^--N) and ammonium-N (NH₄⁺-N); (3) Mild acid-oxidation yielding NH₄⁺-N; (4) the Illinois soil N test (ISNT), measuring amino sugars & NH₄⁺-N concentrations; and (5&6) Ultraviolet (UV) absorbance of 1M KCl filtered soil extract at 260nm and 210nm, respectively.

The 37 soils showed a large range in N_o from 92.17 to 723.08 mg NH₄⁺-N kg^-l. The ISNT had the strongest relationship (R² 0.69) with AI-7 values, followed by UV 260nm (R² 0.38), UV 210nm (R² 0.31) and hot 2M KCl (R² 0.24). There was no significant relationship between the acid oxidation test or the cold 2M KCl test and AI-7. The large range in soil N_o indicates the need to allow for this N in making fertiliser recommendations. These results and the potential techniques to quantify N_o for a range of soil types will be discussed in further detail in this paper.