Journal Article N762921K75 Nitrogen Soil Ammonium Compounds Nitrates 15N isotope dilution 15N isotopic pool dilution technique Clay Meta-Analysis Nitrogen Oxides T1FAD4SS2M climate climate change conservative nitrogen cycle drought drylands ecosystem nitrogen cycling global change land use change land‐use change leaky nitrogen cycle leaky or conservative nitrogen cycle microbial biomass nitrification capacity nitrogen cycle nitrogen deposition scenarios nitrogen loss nitrogen retention soil N retention