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 μm, 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 (SOC_BS and POC_BS, 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, SOC_BS, POC_BS, AN_BS, SOC (SOC_MA), and POC (POC_MA) 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 SOC_BS (R² 0.75, 0.48, and 0.61 at 0–5, 5–20 and 0–20 cm depths, respectively), POC_BS (R² 0.66, 0.31, and 0.49, respectively), and AS (R² 0.80, 0.68, and 0.76, respectively). The AN_MA performed similarly to predict SOC_BS, POC_BS, 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.