Soil-mixing effects on inorganic nitrogen production and consumption in forest and shrubland soils

Soils that are physically disturbed are often reported to show net nitrification and NO3- loss. To investigate the response of soil N cycling rates to soil mixing, we assayed gross rates of mineralization, nitrification, NH4+ consumption, and NO3- consumption in a suite of soils from eleven woody plant communities in Oregon, New Mexico, and Utah. Results suggest that the common response of net NO3- flux from disturbed soils is not a straightforward response of increased gross nitrification, but instead may be due to the balance of several factors. While mineralization and NH4+ assimilation were higher in mixed than intact cores, NO3- consumption declined. Mean net nitrification was 0.12 mg N kg(-1) d(-1) in disturbed cores, which was significantly higher than in intact cores (-0.19 mg N kg(-1) d(-1)). However, higher net nitrification rates in disturbed soils were due to the suppression of NO3- consumption, rather than an increase in nitrification. Our results suggest that at least in the short term, disturbance may significantly increase NO3- flux at the ecosystem level, and that N cycling rates measured in core studies employing mixed soils may not be representative of rates in undisturbed soils.