Soil nitrogen dynamics during stand development after clear-cutting of Japanese cedar (Cryptomeria japonica) plantations

We examined soil N dynamics, including inorganic N concentration, net N transformation rates, and estimated plant N uptake (EPNU) from soil N budgets, and litterfall inputs, in five Japanese cedar plantation stands of different ages (5, 16, 31, 42, and 89 years) in the Mt Gomadan Experimental Forest (GEF). Net soil N mineralization and nitrification rates did not differ significantly between the youngest and oldest stands; soil moisture and inorganic N concentration were higher in the youngest stand. The EPNU was highest in the 16-year-old stand and lowest in the 31-year-old stand, and had a significant negative correlation with litter C:N ratio. The oldest (89-year-old) stand had a higher soil C:N ratio, lower proportion of nitrification rate to mineralization rate (%NIT), and higher estimated plant NH4 + uptake than did the other stands, indicating that changes of soil organic matter quality can alter soil N dynamics. These results suggest that as a Japanese cedar plantation develops, soil N dynamics can be altered by the quantity and quality of input litter and soil organic matter, and can generate the imbalance between N supply from soil and N demand by plant

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

[Background and Aims] Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan. [Methods]At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO3[-]-N) use traits, and compared the results with the soil traits. [Results]Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO3[-]-N. Pieris japonica lacks the capacity to use NO3[-]-N as a N source, but other species do use NO3[-]-N. Lindera triloba, whose distribution is unrelated to soil NO3[-]-N availability, changes the extent to which it uses NO3[-]-N in response to soil NO3[-]-N availability. [Conclusions]Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO3[-]-N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions

The Potential of NO 3

Responses of seedlings of a shrub species, Lindera triloba, grown in perlite culture medium, to nitrate (NO3–-N) supply were investigated to estimate the saturating point of available NO3–-N for plant utilization. NO3–-N concentration and nitrate reductase activity (NRA) in leaves and roots were used as indicators of NO3–-N uptake and assimilation by L. triloba. Root NRA increased with NO3–-N supply when concentrations were low and reached a plateau at high NO3–-N concentrations. On the other hand, root NO3–-N concentration increased linearly with NO3–-N supply; therefore, it is suggested that NO3–-N uptake did not limit NO3–-N assimilation by L. triloba. In contrast, leaf NRA and leaf NO3–-N concentration were low and were not influenced by NO3–-N supply. This may be caused by the lack of transport of NO3–-N from roots to leaves. The NO3–-N retained in perlite was compared with NO3–-N pool sizes in soils from a forest where L. triloba occurs naturally to estimate the level of NO3–-N availability to plants in the forest soil. The maximum NO3–-N pool size in the forest soil was comparable to concentrations at which root NRA reached a plateau in perlite cultures. These results indicate that soil NO3–-N availability is below the saturation point for NO3–-N uptake by L. triloba, and it is the limiting factor of NO3–-N utilization by L. trilobaunder field conditions in which this species naturally occurs

Soil N Fluxes in Three Contrasting Dry Tropical Forests

A comparative study of N fluxes in soil among a dry dipterocarp forest (DDF), a dry evergreen forest (DEF), and a hill evergreen forest (HEF) in Thailand was done. N fluxes in soil were estimated using an ion exchange resin core method and a buried bag method. Soil C and N pools were 38 C Mg/ha/30 cm and 2.5 N Mg/ha/30 cm in DDF, 82 C Mg/ha/30 cm and 6.2 N Mg/ha/30 cm in DEF, and 167 C Mg/ha/30 cm and 9.3 N Mg/ha/30 cm in HEF. Low C concentration in the DDF and DEF sites was compensated by high fine soil content. In the highly weathered tropical soil, fine soil content seemed to be important for C accumulation. Temporal and vertical fluctuations of N fluxes were different among the sites. The highest N flux was exhibited at the onset of the wet season in DDF, whereas inorganic N production and estimated uptake of N were relatively stable during the wet season in DEF and HEF. It is suggested that N cycling in soil becomes stable in dry tropical forests to intermediate in temperate forests. N deposition may result in large changes of N cycling in the DDF and DEF due to low accumulations of C and N

Natural 15 N Abundance of Plants and Soil N in a Temperate Coniferous Forest

Measurement of nitrogen isotopic composition (δ 15 N) of plants and soil nitrogen might allow the characteristics of N transformation in an ecosystem to be detected. We tested the measurement of δ 15 N for its ability to provide a picture of N dynamics at the ecosystem level by doing a simple comparison of δ 15 N between soil N pools and plants, and by using an existing model. δ 15 N of plants and soil N was measured together with foliar nitrate reductase activity (NRA) and the foliar NO 3 – pool at two sites with different nitrification rates in a temperature forest in Japan. δ 15 N of plants was similar to that of soil NO 3 – in the high-nitrification site. Because of high foliar NRA and the large foliar NO 3 – pool at this site, we concluded that plant δ 15 N indicated a great reliance of plants on soil NO 3 – there. However, many δ 15 N of soil N overlapped each other at the other site, and δ 15 N could not provide definitive evidence of the N source. The existing model was verified by measured δ 15 N of soil inorganic N and it explained the variations of plant δ 15 N between the two sites in the context of relative importance of nitrification, but more information about isotopic fractionations during plant N uptake is required for quantitative discussions about the plant N source. The model applied here can provide a basis to compare δ 15 N signatures from different ecosystems and to understand N dynamics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41374/1/10021_2002_Article_132.pd

Sources of uncertainty in estimating stream solute export from headwater catchments at three sites

Uncertainty in the estimation of hydrologic export of solutes has never been fully evaluated at the scale of a small-watershed ecosystem. We used data from the Gomadansan Experimental Forest, Japan, Hubbard Brook Experimental Forest, USA, and Coweeta Hydrologic Laboratory, USA, to evaluate many sources of uncertainty, including the precision and accuracy of measurements, selection of models, and spatial and temporal variation. Uncertainty in the analysis of stream chemistry samples was generally small but could be large in relative terms for solutes near detection limits, as is common for ammonium and phosphate in forested catchments. Instantaneous flow deviated from the theoretical curve relating height to discharge by up to 10% at Hubbard Brook, but the resulting corrections to the theoretical curve generally amounted to \u3c0.5% of annual flows. Calibrations were limited to low flows; uncertainties at high flows were not evaluated because of the difficulties in performing calibrations during events. However, high flows likely contribute more uncertainty to annual flows because of the greater volume of water that is exported during these events. Uncertainty in catchment area was as much as 5%, based on a comparison of digital elevation maps with ground surveys. Three different interpolation methods are used at the three sites to combine periodic chemistry samples with streamflow to calculate fluxes. The three methods differed by \u3c5% in annual export calculations for calcium, but up to 12% for nitrate exports, when applied to a stream at Hubbard Brook for 1997–2008; nitrate has higher weekly variation at this site. Natural variation was larger than most other sources of uncertainty. Specifically, coefficients of variation across streams or across years, within site, for runoff and weighted annual concentrations of calcium, magnesium, potassium, sodium, sulphate, chloride, and silicate ranged from 5 to 50% and were even higher for nitrate. Uncertainty analysis can be used to guide efforts to improve confidence in estimated stream fluxes and also to optimize design of monitoring programmes

Impacts of moso bamboo (Phyllostachys pubescens) invasion on dry matter and carbon and nitrogen stocks in a broad-leaved secondary forest located in Kyoto, western Japan

In western and central Japan, the expansion of exotic moso bamboo (Phyllostachys pubescens Mazel ex J. Houz.) populations into neighboring vegetation has become a serious problem. Although the effects of bamboo invasion on biodiversity have been well studied, shifts in nutrient stocks and cycling, which are fundamental for ecosystem functioning, are not fully understood. To explore the effects of P. pubescens invasion on ecosystem functions we examined above- and below-ground dry matter and carbon (C) and nitrogen (N) stocks in a pure broad-leaved tree stand, a pure bamboo stand, and two tree–bamboo mixed stands with different vegetation mix ratios in the secondary forest of Kyoto, western Japan. In the process of invasion, bamboo shoots offset broad-leaved tree deaths; thus, no clear trend was apparent in total above- or below-ground biomass or in plant C and N stocks during invasion. However, the ratio of above-ground to below-ground biomass (T/R ratio at the stand level) decreased with increasing bamboo dominance, especially in the early stages of invasion. This shift indicates that rapid bamboo rhizomatous growth is a main driver of substantial changes in stand structure. We also detected rises in the C/N ratio of forest-floor organic matter during bamboo invasion. Thus major impacts of P. pubescens invasion into broad-leaved forests include not only early shifts in biomass allocation, but also changes in the distribution pattern of C and N stored in plants and soil

Seasonal Effects on Microbial Community Structure and Nitrogen Dynamics in Temperate Forest Soil

The soil microbial community and nitrogen (N) dynamics change seasonally due to several factors. The microbial community structure (MCS) can regulate N dynamics. However, there is insufficient information on seasonal changes in MCS and the relationship between MCS and N dynamics. We investigated MCS and N dynamics in forest soils with two different fertilities throughout a year. MCS, measured with phospholipid fatty acid (PLFA) analysis, showed a consistent seasonal trend, regardless of the fertility. Microbial indices (particularly the Saturated-/monounsaturated-PLFA ratio; Sat/mono) indicated a major PLFA shift among seasons, with temperature likely the most important factor. The fungal-/bacterial-PLFA ratio in the dormant season (December–April) was approximately 1.3 times greater than in the growing season (June–November). The trend in N dynamics showed that in summer (June–August), the gross N mineralization potential was greater than immobilization, whereas in winter (December–April), immobilization was dominant. The net mineralization potential in the growing season was approximately 1.6 times higher than in the dormant season. Moreover, a relationship was found between Sat/mono and N transformation potentials. We highlight the microbial sensitivity to seasonal dynamics which can be associated with temperature, as well as carbon and N dynamics

Causal Relationships between Stream NO3- Concentrations at Baseflow Conditions and Key Regulating Factors in the Kinki Region of Japan, Including the Japan-sea Side Area and Encompassing a Climatic Gradient

Large nitrate (NO3- leaching from forests is undesirable. Along the Japan-sea side, stream NO3- concentrations are low level in Japan despite the high level of nitrogen (N) deposition. Here, the causal relationships between stream NO3∁Econcentrations and key regulating factors were investigated using structural equation modeling (SEM) for the Kinki region (KIN) of Japan, which consists of seven prefectures and includes the Japan-sea side of KIN (JSK), with the objectives of identifying the specific mechanism of NO3- leaching for the JSK and comparing it with the one for the entire KIN. In the previous study, stream water was collected from 1,691 watersheds without human-created features in KIN (405 from JSK) between 1997 and 2012, and key factors regulating NO3- leaching were clarified using Random Forest regression. SEM mostly supported the previously suggested mechanisms. Deposited N was incorporated into the ecosystem, enhanced N mineralization and nitrification in the soil, and increased NO3- leaching. Heavy rain over the entire KIN and snow along JSK lowered NO3- concentration in soil water and NO3- leaching. Contrary to the expectation, precipitation during the growing season along JSK contributed to N input into the ecosystems. We provided the first statistical mechanism of NO3- leaching using SEM