Net and gross N transformation rates in subtropical forest soils under aerobic and anaerobic conditions

Abstract

Purpose This study investigated the differences between net and gross rates of N transformation in subtropical forest soils under aerobic and anaerobic conditions. We aim to explore the factors responsible for the differences observed and to evaluate the ability of net and gross N transformation measurements to reflect the occurrence and stimulation of N transformation in subtropical forest soils. Materials and methods Subtropical soil samples from adjacent native forest (NF) and two plantation forest sites (Pinus caribaea var. hondurensis (PCH) and Hoop pine (HP), Araucaria cunninghammii), with different initial pool sizes of NH 4 + and NO 3 − , were collected from South East Queensland, Australia. Soil samples were incubated at 25°C to determine net and gross N transformation rates ( 15 N pool dilution method) under aerobic and anaerobic conditions. Results and discussion Under aerobic and anaerobic conditions, gross N transformation rates differed significantly from net N transformation rates. NH 4 + immobilization occurred under aerobic and anaerobic conditions for all soils. NO 3 − immobilization occurred under aerobic conditions in the NF soils but was negligible in both the PCH and HP soils. Gross ammonification rates were significantly correlated with the C/N ratios (R 2 =0.714, p<0.001). Gross nitrification rates were significantly correlated with soil carbon composition ( 13 C, R 2 =0.569, p<0.01). The ratios of the gross rates of NH 4 + immobilization to the gross ammonification rates (R_i a /g a ) were significantly correlated with δ 13 C (R 2 =0.829, p<0.001). The results indicated that in the subtropical forest soils, both the variability of gross N production and immobilization mainly influenced the net N transformation rates. Forest type affected the gross production or immobilization of NH 4 + and NO 3 − via C turnover, which was indicated by the relationship between the N transformation rates and soil C/N ratio or soil 13 C.Conclusions Although subtropical forest soils are generally an N-unlimited system, the immobilization of NH 4 + and NO 3 − under aerobic or anaerobic conditions played an important role in the N transformations. Net rates of NH 4 + /NO 3 − accumulation are not suitable to describe the occurrence and stimulation of N transformations in subtropical soils when immobilization (or other consumption processes) of NH 4 + and NO 3 − is occurring, especially under anaerobic conditions.

Nitrogen mineralization and immobilization in sediments of the East China Sea: Spatiotemporal variations and environmental implications

“…In addition, sediment C:N ratio negatively covaried with both GNM and GAI rates in the ECS (Table ), indicating that TOC available contents and C:N stoichiometry might play an important role in sediment N mineralization and N immobilization in this ecosystem. A similar relationship between GNM rate and C:N ratio has been reported in other ecosystems [ Zhu et al , ; Regehr et al , ; Zhao et al , ]. Hence, the spatial patterns of N mineralization and immobilization in the study area were remarkably affected by the bioavailability of TOC and TN.…”

“…Nevertheless, microbes preferentially assimilate NH 4 + due to higher energy costs associated with biological NO 3 − assimilation, and thus, the presence of NH 4 + may greatly inhibit microbial NO 3 − immobilization [ Zhu et al , ]. Meanwhile, NO 3 − immobilization plays a negligible role in total microbial N immobilization under anaerobic conditions [ Zhao et al , ]. Moreover, numerous studies have demonstrated that microbial NH 4 + immobilization is the dominant process of NH 4 + consumption [ Di et al , ; Bengtsson et al , ; Bedard‐Haughn et al , ; Gütlein et al , ], which can effectively remove the accumulated NH 4 + in sediments and maintain ecological health [ Matheson et al , ].…”

Soil gross N ammonification and nitrification from tropical to temperate forests in eastern China

“…Thus, together with the results of previous studies, our data indicate that the possible influence of climate gradients might be masked by the large spatial heterogeneity in gross N turnover in forest ecosystems (Burton, Chen, Xu, & Ghadiri, 2007). With regard to the magnitudes of GA and GN, Zhao, Cai, and Xu (2015) reported rates of GA and GN of 5.00 mg kg −1 day −1 and up to 2.19 mg kg −1 day −1 , respectively, for subtropical forests in China. A further study by Zeng et al (2014) has found comparably high rates, that is, GN rates of 4, 5.3 and 6 mg kg −1 day −1 in coniferous, deciduous and evergreen broad- ranging from 2.5 to 8.6 mg kg −1 day −1 , and GN rates in the range of 0.6-3.49 mg kg −1 day −1 .…”

“…Thus, together with the results of previous studies, our data indicate that the possible influence of climate gradients might be masked by the large spatial heterogeneity in gross N turnover in forest ecosystems (Burton, Chen, Xu, & Ghadiri, ). With regard to the magnitudes of GA and GN, Zhao, Cai, and Xu () reported rates of GA and GN of 5.00 mg kg −1 day −1 and up to 2.19 mg kg −1 day −1 , respectively, for subtropical forests in China. A further study by Zeng et al.…”