Factors contributing to soil nitrogen mineralization and nitrification rates of forest soils in the Japanese archipelago
Introduction
Soil nitrogen (N) transformations (i.e., N mineralization and nitrification) are key processes regulating ecosystem services such as forest productivity, soil fertility and streamwater quality, because N is a limiting nutrient in forest ecosystems (Vitousek and Howarth, 1991, Mitchell, 2011). Understanding how environmental factors influence N transformations is essential for the provision of sustainable ecosystem services, especially in a forest ecosystem.
Although there are a number of studies that considered the effects of various environmental factors on N transformations, the major controlling factors changed among studies according to the scale and spatial range of the study sites (Aerts, 1997). For example, at larger spatial scales up to the global scale, climatic conditions and substrate amounts were significant controlling factors (Booth et al., 2005, Colman and Schimel, 2013). At smaller scales, site characteristics such as vegetation type (Hobbie, 1992, Hobbie, 1996, Hobbie, 2015), soil acidity (Cookson et al., 2007, Tian et al., 2012), salinity (McClung and Frankenberger, 1985, Noe et al., 2013) and soil water content (Gutiérrez-Girón et al., 2014) become more important. Some of these factors are interlinked, such that some factors directly control N transformations, while others mediate them by affecting the direct factors (Binkley and Giardina, 1998). For example, soil organic matter content, which directly affects N transformation rates (Booth et al., 2005, Cookson et al., 2007), is affected by organic matter decomposition in the litter layer (e.g. Berg and McClaugherty, 2008). Decomposition of the litter layer is influenced by the quantity and quality of the litter layer (Coûteaux et al., 1995, Fierer et al., 2005), and litter layer characteristics are constrained by vegetation type (Hobbie, 1992, Hobbie, 1996, Ono et al., 2013) and climatic conditions (Coûteaux et al., 1995, Aerts, 1997, Inagaki et al., 2010). Therefore, it is important to consider both direct and indirect cause–effect structural relationships to better understand the factors contributing to N transformations. For this analysis, a wide ranging dataset containing geographical locations, N transformation rates and comprehensive environmental and soil characteristics is absolutely necessary. However, there are few published large scale studies based on exhaustive surveys (Colman and Schimel, 2013) or literature surveys (Booth et al., 2005) dealing with N transformation rates and related environmental and soil characteristics. Previous studies conducted regression analysis (Booth et al., 2005) and structural equation modeling (SEM; Colman and Schimel, 2013) to clarify the effects of environmental factors on N transformation rates. To conduct structural analysis of cause–effect relationships surrounding N transformations, a database containing a wide range of items obtained with unified protocols through many survey points is essential. To our knowledge, in East Asia, and in Japan particularly, there is no suitable information source or database available for such intensive statistical analysis. Although there have been two previous large-scale ecosystem surveys in Japan [Forest Soil Survey (Editorial committee of “Forest soils in Japan”, 1983) and Japan-International Biological Program (Kira et al., 1978)], these surveys did not consider the possible secondary uses of the datasets; they contain limited information on nutrient cycling in Japanese forest ecosystems.
The Japanese archipelago ranges approximately 3000 km from north to south, and the climatic zones ranges from cool temperate to subtropical. The biodiversity of forest ecosystems of Japan is considerably high because of the wide range of its geographical and climatic distribution and the diverse use-history of the forest (Ministry of the Environment, 2010). Thus, soil N transformations in Japanese forest ecosystems are likely to be influenced by these conditions.
Andosols/Andisols (IUSS Working Group WRB, 2014, Soil Survey Staff, 2014), which are generated from volcanic sediments, are broadly distributed throughout forest soils in Japan (Nanko et al., 2014). Andosols/Andisols are very specific soil type characterized by a high organic matter content (Batjes, 1996, Imaya et al., 2010), low bulk density (Nanko et al., 2014), and high activated aluminum (Al) content (Wada, 1986). It has been suggested that soil group/soil parent material significantly affects N dynamics in forest ecosystems (Reich et al., 1997). Therefore, soil N transformations in Japanese forest ecosystems might be influenced by soil parent materials, as identified by soil groups, and the factors affecting N transformations might be different among soil groups.
Recently in Japan, there has been increasing concern about the effects of climate change (IPCC, 2007) and the increase in cross-border air pollutants from the continent (Ministry of the Environment, 2014) on nutrient cycling in forest ecosystems. Although applying ecosystem models is efficient to evaluate and predict these effects, large-scale ecosystem databases, which are crucial for validating these ecosystem models, are very limited. To solve this situation, a database was recently developed on the biogeochemical N properties of forest soils in the Japanese archipelago (Urakawa et al., 2015). This database contains environmental items such as mean annual temperature and precipitation, dominant tree species and physicochemical soil properties at approximately 40 sites across Japan. The database also contains both net and gross rates of N mineralization and nitrification, measured independently by laboratory incubation for 4 weeks at constant temperature for net rates and by the 15N isotope dilution method for gross rates. Gross rates, which reflect microbial activities, are a more direct and precise index than net rates when discussing N transformations (Davidson et al., 1992, Schimel and Bennett, 2004). It is common to observe substantial rates of gross mineralization and nitrification while small net rates are observed, because net rates only account for the net differences in NH4+ and NO3− concentrations for a given time interval (Hart et al., 1994b). On the other hand, net N transformation rates still provide a useful index of plant N availability (Davidson et al., 1992, Schimel and Bennett, 2004), providing important information for evaluating soil fertility and ecosystem services. However, there are few studies in which both gross and N transformation measurements have been conducted independently (Booth et al., 2005). It is meaningful to compare N transformation rates, which are considered to be influenced by ecological and pedological characteristics of Japan, with those of other temperate forest ecosystems.
The objectives of this study were (i) to clarify the characteristics of net and gross rates of N transformation in forest soils in Japan; and (ii) to elucidate the structural cause–effect relationships of N transformation rates and associated environmental factors, especially considering the effect of soil parent materials, which are thought to have significant effects on N transformations in Japanese forest ecosystems through the specific characteristics of Andosols. To achieve these objectives, we evaluated the range and relationships among items using the dataset of biogeochemical N properties of forest soils in the Japanese archipelago (Urakawa et al., 2015). We conducted SEM analysis to clarify the effect of environmental factors including climatic conditions, vegetation type, organic layer conditions, and soil pedological and physicochemical properties on gross and net N transformation rates as important indices representing ecosystem services. We also assessed whether the factors contributing to N transformation rates changed among soil groups/soil parent materials.
Section snippets
Materials and methods
All data used for analyses in this study were obtained from the database developed by Urakawa et al. (2015), and the protocols for soil sampling, sample preparation, laboratory analyses and data compilation are precisely described in the JaLTER database (http://db.cger.nies.go.jp/JaLTER/ER_DataPapers/archives/2014/ERDP-2014-02). The study methods are briefly described below.
Results
The average amount of the total organic layer was 16.3 ± 14.8 Mg ha−1 (average ± S.D.; Table 2) and large values were observed in sites where L. kaempferi was planted (Table 1). The average organic layer C/N was 45.7 ± 21.4, and was significantly higher in two evergreen coniferous species (C. obtusa and C. japonica) stands, which are typical species in planted forests in Japan (Fig. 4a). Total C (TC) of the surface soil layer ranged from 1.7% to 19.6% and total N (TN) ranged from 0.11% to 0.97% (Table 2
Characteristics of N transformation rates in Japanese forest soils
Net N transformation rates that are measured by relatively long-term incubation (∼30 days) are considered to provide a useful index of plant N availability (Hart et al., 1994, Schimel and Bennett, 2004), because these rates indicate the residual inorganic N subtracted by the consumption of soil microbes. The average net N mineralization and nitrification rates in the Japanese forest soils were 0.62 ± 0.68 and 0.59 ± 0.65 mg N kg−1 d−1, respectively, and the sites that had higher rates of both N
Conclusions
N transformation rates in 38 forest sites across Japan were almost within the ranges of temperate forest ecosystems obtained by intensive large scale surveys (Booth et al., 2005, Colman and Schimel, 2013), suggesting that N transformation rates in the Japanese forest ecosystem were representative of temperate forest ecosystems and considerably diverse despite the relatively narrow land area. To elucidate the structural cause–effect relationships among N transformations, we conducted SEM
Acknowledgements
This study was conducted as part of the GRENE (Green Network of Excellence) environmental information project (PI: Motomi Itoh, University of Tokyo) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was also supported by the ReSIN-III (Regional and comparative Soil Incubation study on Nitrogen dynamics in forest ecosystems-III) project and Grant-in-Aid for Scientific Research funded by the Japan Society for the Promotion of Science (25252026, 26660127 and
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