Anaerobically mineralized nitrogen within macroaggregates as a soil health indicator

Anaerobically mineralized nitrogen (AN) in bulk soil (AN(BS)) has been described as a soil health indicator. Considering that large macroaggregates (2000-8000 mu m, MA) are more sensitive to management practices than the bulk soil (i.e. whole soil), AN within MA (AN(MA)) would be a better soil health indicator than AN(BS). The aim of this study was to evaluate if AN(MA) is a better indicator of: i) soil organic carbon (SOC) and particulate organic carbon (POC) in bulk soil (SOCBS and POCBS, respectively) and ii) aggregate stability (AS) than AN(BS). Soil samples were taken at 0-5 and 5-20 cm from 46 continuously cultivated plots (CC) and a reference plot for each CC (pseudo-pristine, PRIS). These soils, located in the Argentinean Pampas, were classified as Mollisols with contrasting surface textural classes. The AS, SOCBS, POCBS, AN(BS), SOC (SOCMA), and POC (POCMA) within MA and AN(MA) were determined separately at 0-5 and 5-20 cm soil depths and estimated at the 0-20 cm layer. The AN(MA) was a good indicator of SOCBS (R-2 0.75, 0.48, and 0.61 at 0-5, 5-20 and 0-20 cm depths, respectively), POCBS (R-2 0.66, 0.31, and 0.49, respectively), and AS (R-2 0.80, 0.68, and 0.76, respectively). The AN(MA) performed similarly to predict SOCBS, POCBS, and AS as compared to AN(BS), because AN(MA) was closely correlated to AN(BS) (r 0.90 at 0-20 cm). Since AN(MA) determination is more time-consuming than AN(BS) determination, its use as a soil health indicator would not be convenient. Therefore, the use of AN(BS) would be recommended over AN(MA) as a variable to monitor soil health.