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N fertilization effects on denitrification and N cycling in an aggrading forest

机译：氮肥对退化森林中反硝化和氮循环的影响

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We investigated N cycling and denitrification rates following five years of N and dolomite amendments to whole-tree harvested forest plots at the long-term soil productivity experiment in the Fernow Experimental Forest in West Virginia, USA. We hypothesized that changes in soil chemistry and nutrient cycling induced by N fertilization would increase denitrification rates and the N2O:N-2 ratio. Soils from the fertilized plots had a lower pH (2.96) than control plots (3.22) and plots that received fertilizer and dolomite (3.41). There were no significant differences in soil %C or %N between treatments. Chloroform-labile microbial biomass carbon was lower in fertilized plots compared to control plots, though this trend was not significant. Extractable soil NO3- was elevated in fertilized plots on each sample date. Soil-extractable NH4+, NO3-, pH, microbial biomass carbon, and %C varied significantly by sample date suggesting important seasonal patterns in soil chemistry and N cycling. In particular, the steep decline in extractable NH4+ during the growing season is consistent with the high N demands of a regenerating forest. Net N mineralization and nitrification also varied by date but were not affected by the fertilization and dolomite treatments. In a laboratory experiment, denitrification was stimulated by NO3- additions in soils collected from all field plots, but this effect was stronger in soils from the unfertilized control plots, suggesting that chronic N fertilization has partially alleviated a NO3- limitation on denitrification rates. Dextrose stimulated denitrification only in the whole-tree-harvest soils. Denitrification enzyme activity varied by sample date and was elevated in fertilized plots for soil collected in July 2000 and June 2001. There were no detectable treatment effects on N2O or N-2 flux from soils under anaerobic conditions, though there was strong temporal variation. These results suggest that whole-tree harvesting has altered the N status of these soils so they are less prone to N saturation than more mature forests. It is likely that N losses associated with the initial harvest and high N demand by aggrading vegetation is minimizing, at least temporarily, the amount of inorganic N available for nitrification and denitrification, even in the fertilized plots in this experiment.
机译：我们在美国西弗吉尼亚州的费尔诺实验森林进行了长期土壤生产力实验，调查了整棵树采伐的森林中对氮和白云石进行五年修正后的氮循环和反硝化率。我们假设氮肥引起的土壤化学和养分循环的变化会增加反硝化率和N2O：N-2的比率。施肥区的土壤的pH值（2.96）低于对照区（3.22）和接受肥料和白云石的区（3.41）。处理之间的土壤％C或％N无显着差异。受精地块的氯仿不稳定性微生物生物量碳比对照地块低，尽管这种趋势并不明显。在每个采样日期，施肥区的可提取土壤NO3-均升高。土壤中可提取的NH4 +，NO3-，pH，微生物生物量碳和％C随采样日期而变化很大，表明土壤化学和氮循环过程中重要的季节性模式。特别是在生长季节，可提取的NH4 +急剧下降与再生森林对氮的高需求相一致。净氮矿化和硝化作用也随日期变化，但不受施肥和白云石处理的影响。在实验室实验中，从所有田间地块收集的土壤中添加了NO3-刺激了反硝化作用，但在未施肥的对照地块中的土壤中这种作用更强，表明长期施氮已部分缓解了NO3-对反硝化率的限制。葡萄糖仅在整棵树收获的土壤中刺激反硝化作用。在2000年7月和2001年6月采集的土壤施肥地中，反硝化酶的活性有所变化，并有所升高。在厌氧条件下，土壤中的N2O或N-2通量没有明显的处理效果，尽管时间变化很大。这些结果表明，整棵树的采伐改变了这些土壤的氮素状况，因此与成熟的森林相比，它们不容易氮饱和。即使是在本试验的施肥地中，与初期收获有关的氮损失和由于植被退化造成的高氮需求也可能（至少暂时）使可用于硝化和反硝化的无机氮量最小化。

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