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

Abstract

Soils that are physically disturbed are often reported to show net nitrification and NO 3 -loss. To investigate the response of soil N cycling rates to soil mixing, we assayed gross rates of mineralization, nitrification, NH 4 + consumption, and NO 3 -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 NO 3 -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 NH 4 + assimilation were higher in mixed than intact cores, NO 3 -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 NO 3 -consumption, rather than an increase in nitrification. Our results suggest that at least in the short term, disturbance may significantly increase NO 3 -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.

Cross-biome assessment of gross soil nitrogen cycling in California ecosystems

“…Soil mixing can alter gross rates of N mineralization and nitrification (Booth et al, 2006;Kaur et al, 2010;Schimel et al, 1989). It is thought that mixing breaks up soil aggregates to release physically protected organic matter, thereby increasing substrate availability for mineralization to stimulate gross N mineralization rates (Booth et al, 2006;Schimel et al, 1989). The gross N mineralization rates we observed were within the range of values reported in the Booth et al (2005) meta-analysis, which found no significant difference in rates documented in studies using intact versus mixed soils.…”

“…Third, we used gently mixed (i.e., not sieved) soil for the assays to allow us to relate multiple N cycling processes and soil properties to the same homogenous soil sample, facilitating our objective to correlate these variables with each other. Soil mixing can alter gross rates of N mineralization and nitrification (Booth et al, 2006;Kaur et al, 2010;Schimel et al, 1989). It is thought that mixing breaks up soil aggregates to release physically protected organic matter, thereby increasing substrate availability for mineralization to stimulate gross N mineralization rates (Booth et al, 2006;Schimel et al, 1989).…”

“…This suggests that using mixed soils does not consistently bias measured gross N mineralization rates higher or lower than rates measured in intact soils. The mixing of soil also homogenizes the distribution of organic matter and could potentially inhibit nitrification by favoring heterotrophs that compete better for NH 4 þ in C-rich environments (Booth et al, 2006). However, there is no evidence of this effect (Booth et al, 2006;Kaur et al, 2010).…”

“…We note that short-term laboratory assays may not reflect long-term field rates of gross N cycling (i.e., months to years), especially because of seasonality in field conditions. Disturbance effects of mixing soils for the laboratory assays may also change soil structure and substrate availability to alter rates of N cycling (Booth et al, 2006;Kaur et al, 2010;Schimel et al, 1989). Our goal here was to compare laboratory rates among soils from different ecosystems while maintaining soil moisture and substrate availability for the various N cycling processes similar to field conditions at the time of soil collection.…”

Artefacts of the pot environment on soil nutrient availability: implications for the interpretation of ecological studies

“…Nitrate production can be enhanced by soil disturbance, e.g., through tillage (Kristensen et al 2000) or when soil is placed inside cores (Ross and Hales 2003;Booth et al 2006). Simply collecting soil samples for later chemical analysis can also result in considerable increases in ammonium and nitrate concentrations within hours of removal from the field (Turner and Romero 2009).…”