Global Land Use Change Impacts on Soil Nitrogen Availability and Environmental Losses

Anthropogenic activities, particularly land use change
and management
practices, alter the global nitrogen (N) cycle. As a central part
of the global N cycle, soil N supply from net N mineralization (NNM)
and net nitrification (NN) contributes to over 50% of crop N uptake.
However, how global land use changes impact soil N supply and potential
N loss remains elusive. By compiling a global data set of 1,782 paired
observations from 185 publications, we show that land use conversion
from natural to managed ecosystems significantly reduced NNM by 7.5%
(−11.5, −2.8%) and increased NN by 150% (86, 194%),
indicating decreasing N availability while increasing potential N
loss through denitrification and nitrate leaching. In contrast, reversing
managed to natural ecosystems significantly increased NNM by 20% (9.7,
25.4%) and decreased NN by 89% (−125, −46%), indicating
increasing N availability while decreasing potential N loss. Structural
equation modeling revealed that land use change induced changes in
soil properties, including organic matter content, bulk density, microbial
biomass and pH, and anthropogenic activities, including application
of ammonium-based fertilizers and manure, were the most important
factors regulating NNM and NN. The land use change effect was the
strongest in tropical and subtropical regions, where NNM was negatively
affected and NN positively affected by land use change. Our findings
indicate that increasing soil organic matter content and enhancing
soil structural development post land use change can boost soil N
supply and reduce the risk for N loss.