Factors controlling net soil nitrogen mineralization rates in Japanese forest ecosystems

Abstract

Background and aimsNet soil nitrogen (N) mineralization rate represents bioavailable N supply rate. Exploring its controlling factors improves the knowledge of N cycling. Previous studies in Japanese forests revealed that high soil organic matter (SOM) content of volcanic soils enhanced net N mineralization rates. However, the effects of cations and phosphorus (P) added by volcanic ash are not fully understood. We investigated the influences of volcanic ash addition on net N mineralization via P and cation availabilities.MethodsNet soil N mineralization rates of surface soils (0-5 cm) collected from 20 sites across Japan were determined under the common laboratory condition (20 ℃, 10 days). The concentration of amorphous minerals (Alo+1/2Feo; a surrogate of volcanic ash) was included as an independent variable as a surrogate of volcanic ash in a structural equation modelling (SEM). In addition, conifer abundances and mean annual temperatures of the origins of soils were also included in the model.ResultsNet soil N mineralization rates differed among sites, suggesting that soil substrates were inherently determined by site conditions. Alo+1/2Feo positively affected net soil N mineralization rate via the concentrations of total P and labile P. Conifer abundance weakly affected net soil N mineralization rate. Temperature was not selected in the model.ConclusionsNet soil N mineralization rates in Japanese forest ecosystems are most strongly determined by the amount of added volcanic ash via P availability. These findings improve our understanding for the diversification of forest ecosystems under volcanic activities.

Variations in soil nutrient availabilities and foliar nutrient concentrations of trees between temperate monsoon karst and non‐karst forest ecosystems on Mount Ibuki in Japan

“…The lower soil C:N ratio in the karst site is consistent with the results of a recent study conducted at the same sites by Nakamura et al (2023) that the abundance of bacteria in the soil is higher in the karst site than in the non‐karst site. Moreover, as the rates of N mineralization and nitrification increase in the presence of high P availability (DeForest & Otuya 2020; Sasaki et al, 2022), it is also possible that these processes are more active in the limestone site. Such inorganic N could be absorbed by trees more rapidly in the limestone site, and it might result in a similar amount of inorganic N extracted in this study.…”

“…This study hypothesized that karst soils with relatively low pH (approximately between 6.0 and 7.0) but still rich in Ca have high P availabilities. Furthermore, if P availability is high, the N cycling process, such as mineralization or nitrification, may become faster (DeForest & Otuya, 2020; Sasaki et al, 2022). Thus, we also hypothesized that karst soils with low pH but still rich in Ca also have high N availability, although low soil pH generally results in a low soil nitrification rate (Ste‐Marie & Paré, 1999).…”