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Throughfall ( TF ) is a critical component of the hydrological and biogeochemical cycles and is greatly influenced by stand structures in forested watersheds. Previous studies have examined the relationships between TF and stand structures for different species and regions. However, there remains acknowledged difficulty in estimation of TF with changing stand structures due to forest management (e.g., thinning) for specific species and regions. This study conducted intensive thinning with 43–50% stem removal at seven experimental plots with various structures (e.g., stand density, canopy cover and basal area) across Japan that were covered by Japanese cypress ( Chamaecyparis obtusa Endl.) and cedar ( Cryptomeria japonica D. Don) plantations. TF was measured by a set of 20 tipping-bucket rain gauges placed in a lattice-like pattern within the plots before and after thinning. Results showed that during the study periods, thinning caused an increase in the mean TF rate from 63.2 ± 7.0% to 75.4 ± 6.0%. The TF rate was significantly negatively correlated with stand density, canopy cover, and basal area on a stand scale. Additionally, the slope ( a TF ) of event-based linear TF - P g (gross rainfall) equation was significantly higher in post-thinning (0.782 ± 0.077) than that in pre-thinning (0.663 ± 0.078), whereas it’s intercept ( b TF ) showed no significant difference (−0.707 ± 0.810 mm in pre-thinning and −0.441 ± 0.607 mm in post-thinning). The a TF significantly decreased with increasing stand density and canopy cover, whereas the b TF was not significantly correlated with any forest-structure variables. Further, the TF rate and a TF were estimated, respectively, by their related stand-structural variables in multiple regression models with high determination coefficients and moderate relative errors. The b TF was assumed to be a constant and its mean value obtained from all the experimental plots in each (pre- and post-thinning) period. Thus, the models of stand-scale TF rate and event-based TF amount were developed with input of commonly forest inventories. These models were validated by using the dataset of this study and earlier publications for Japanese cedar and cypress plantations, and showed good fit between the estimated and measured values. These models are practical tools that can be readily used for assessing the changes in TF with changing stand structures at both stand and event-based scales, and have also potential implications in evaluating the spatial TF patterns at catchment scale and exploiting similar models in other species and regions. [ABSTRACT FROM AUTHOR]

• Spring wheat was 15N-labeled via a split-root technique. • The NdfR was estimated by the isotope ratio and isotope mass balance approaches. • The isotope ratio approach underestimated NdfR before plant maturity. • The two approaches had no difference for estimation of NdfR after plant maturity. • Selecting appropriate calculation approach to quantify NdfR should be considered. Accurate quantification of nitrogen (N) derived from rhizodeposition (NdfR) is crucial to assess N stored in plants and the total N balance in plant-soil systems. However, there remains uncertainty in the estimates of NdfR for 15N-labeled plants with the split-root technique using the isotope ratio approach and the isotope mass balance approach. Here, we examined the two calculation approaches that estimate NdfR from 15N-urea (99 atom%) labeling of spring wheat (Triticum aestivum L.) using the split-root technique in a pot experiment over plant growth, and assessed the causative factors of their potential bias. The results showed that independent of the approach, the total amount of NdfR was 5.3–9.8 mg plant−1 and contributed up to 19.9–26.3% of total plant N throughout the growth period. At the heading and filling stages after labeling, the amounts of NdfR using the isotope ratio approach (5.3 and 6.6 mg plant−1, respectively) were significantly lower than those (7.1 and 7.8 mg plant−1, respectively) using the isotope mass balance approach, due to soil N losses and an uneven distribution of 15N. At the ripening stage after labeling, however, the differences in the amount of NdfR between the two approaches disappeared (9.8 and 9.6 mg plant−1, respectively). This indicated that the isotope ratio approach underestimates the amount of NdfR during the plant developmental period, while both approaches can be used to quantify the amount of NdfR at plant maturity. [ABSTRACT FROM AUTHOR]

• TF or SF base cations fluxes had no significant differences between UP and LP. • Relative contributions of DD and CE to NTF were different at the two slope plots. • NTF K+ flux was dominated by CE regardless of slope position. • NTF Ca2+ and Mg2+ fluxes were dominated by DD in UP, while CE in LP. • ANC and ALK of TF and SF were significantly higher compared to BP. Atmospheric deposition of base cations in contact with the canopy alters their chemical characteristics and plays a vital role in offsetting soil acidification and nutrient imbalance by replenishing the base cationic pool in forest ecosystems. However, the effects of site characteristics on deliveries of base cations fluxes via throughfall (TF) and stemflow (SF) across topographic gradients within a watershed remain less understood. Here, we examined the concentrations and fluxes of base cations (potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sodium (Na+)), acid neutralizing capacities (ANC) and alkalinity (ALK) in bulk precipitation (BP), TF and SF in an upper slope plot (UP) and a lower slope plot (LP) in a subtropical broadleaved montane forest, Southeast China. In addition, a canopy budget model was applied to distinguish the relative contributions of dry deposition (DD) and canopy exchange (CE) to net TF + SF (NTF) flux. Results showed that annual volume weighted mean concentrations of base cations in TF and SF were significantly higher than those in BP in both UP and LP. Annual fluxes of TF or SF base cations had no significant differences between UP and LP, resulting in a comparable of the NTF fluxes. However, the relative contributions of DD and CE to NTF were different at the two slope plots. NTF K+ flux was dominated by CE regardless of slope position. Unlike K+, NTF Ca2+ and Mg2+ fluxes were dominated by DD in UP, while CE in LP. In addition, ANC and ALK of TF and SF were significantly higher compared to BP , suggesting rainwater beneath the canopy became less acidic and may mitigate in part the risk of soil acidification, especially in the dormant season. This study can provide insights into the spatial variability of chemical characteristics of base cations, and improve our understanding of the quantities and controls of external and internal nutrient inputs occurring within the canopy in forest watershed. [ABSTRACT FROM AUTHOR]

Knowledge of the characteristics of chemical components transported in the xylem sap of trunks remains deficient and limited because no appropriate method exists to extract the xylem sap from this part of the tree. We thus explored the differences in xylem sap components extracted by means of centrifugation and water displacement methods and depicted the level and behavior of chemical components in the xylem sap of trunks and branches of different aged trees from a pine forest in northern China. There were no significant differences between the two methods with respect to nitrogen (N) compounds and inorganic ions in the xylem sap. Potassium concentrations obtained by the methods were similar and consistent with the values obtained from earlier publications on woody species. This suggests that contamination of the xylem sap by the centrifugation method is negligible, and this method would be a reliable and robust tool for collection of the trunk xylem sap. Dissolved organic N was the dominant component of total N followed by nitrate (NO 3 −) and ammonium (NH 4 +). Potassium and chloride were the predominant cation and anion, respectively, of the xylem sap. The NO 3 − concentration basically did not change, whereas the NH 4 + concentration was larger transported from the trunk to branches for the large tree class during foliage senescence. More inorganic N components (mainly NO 3 −) were found in young trees than in old trees. Our study contributes to improve the diagnostic assessments of tree physiological processes and growth in mature forest trees under environmental changes. • A centrifugation method was proposed to collect the trunk xylem sap of forest trees. • DON was the dominant component of TN in trunk xylem sap followed by NO 3 − and NH 4 +. • K+ and Cl− were the predominant cation and anion, respectively, of trunk xylem sap. • NO 3 − and NH 4 + had different behaviors when transported from the trunk to branches. • More inorganic N components (mainly NO 3 −) were found in young trees. [ABSTRACT FROM AUTHOR]

A wide area of paddy fields was contaminated by radiocesium derived from the Fukushima nuclear accident. Implement of agricultural countermeasures is one of the principle methods to reduce the contamination of rice plants. In this study, the effect of topsoil removal measure and fertilizer application on radiocesium uptake by rice plants was investigated over a four-year period. Some other available countermeasures carried out after the Fukushima nuclear accident were also summarized. The results indicate that the effect of topsoil removal measure on the accumulation of radiocesium in rice plants was effective, but the concentration ratio of radiocesium activity concentration between rice plant and soil increased. This may be correlated with the radiocesium imported from irrigation water and relatively high exchangeable radiocesium proportion of plowing soil in the topsoil removal paddy. We summarized four year's data to further confirm that potassium and nitrogen fertilizers had an opposite effect on the accumulation of radiocesium in rice plants. Increasing potassium and reducing nitrogen fertilizer conditions tended to inhibit the radiocesium uptake by rice plants. Moreover, of all the available countermeasures applied in the paddy fields, the most effective countermeasure was the application of phlogopite. However, further analyses are required to confirm the effectiveness of application of phlogopite considering the limited available information. [ABSTRACT FROM AUTHOR]