Abstract
Soil nitrogen (N) processes and inorganic N availability are closely coupled with ecosystem productivity and various ecological processes. Spatio-temporal variations and environmental effects on net N transformation rates and inorganic N concentrations in bulk soil and ion exchange resin were examined in an upland pine forest (UPF) and a bottomland alder forest (BAF), which were expected to have distinguishing N properties. The annual net N mineralization rate and nitrification rate (kg N·ha−1·year−1) were within the ranges of 66.05–84.01 and 56.26–77.61 in the UPF and −17.22–72.24 and 23.98–98.74 in the BAF, respectively. In the BAF, which were assumed as N-rich conditions, the net N mineralization rate was suppressed under NH4+ accumulated soils and was independent from soil temperature. On the other hand, in the UPF, which represent moderately fertile N conditions, net N transformation rates and N availability were dependent to the generally known regulation by soil temperature and soil water content. Stand density might indirectly affect the N transformations, N availability, and ecosystem productivity through different soil moisture conditions. The differing patterns of different inorganic N indices provide useful insight into the N availability in each forest and potential applicability of ion exchange resin assay.
Highlights
Soil nitrogen (N) has become a focus of terrestrial ecosystem studies for two main reasons:(1) N availability is a limiting factor for terrestrial ecosystem productivity [1,2,3] and (2) N losses from terrestrial ecosystems impact the atmosphere especially by greenhouse gas emission, aquatic ecosystems by eutrophication, and drinking water by nitrate contamination [4,5]
NH4+ was abundant at the high moisture (HM) and the middle moisture (MM) bottomland alder forest (BAF) stands (Figure 1a); whereas NO3− was rich at the low moisture (LM) BAF stand (Figure 1b)
Total inorganic N concentration was lowest at the upland pine forest (UPF) stands (Figure 1c)
Summary
Soil nitrogen (N) has become a focus of terrestrial ecosystem studies for two main reasons:(1) N availability is a limiting factor for terrestrial ecosystem productivity [1,2,3] and (2) N losses from terrestrial ecosystems impact the atmosphere especially by greenhouse gas emission, aquatic ecosystems by eutrophication, and drinking water by nitrate contamination [4,5]. Soil nitrogen (N) has become a focus of terrestrial ecosystem studies for two main reasons:. N mineralization, the microbial conversion of organic N to inorganic N, has been intensively studied because it is believed to be the principal control of N availability to plants in terrestrial ecosystems [6]. Environmental controls of N mineralization and availability, temperature and soil moisture, have been investigated through a theoretical model [11], literature review [9,12], laboratory experiments [13,14,15], and field manipulations [16,17,18,19]. Understanding soil moisture and its effect on N and nutrient processes is essential in soil ecology
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