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Aims: Nitrogen (N) deposition alters plant stoichiometry and the state of nutrient limitation in forest ecosystems. In subtropical forests of China, responses of plants stoichiometry to chronically elevated and recently declined N deposition remain unclear. Methods: A 14-year investigation of needle elements, including N, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and aluminum (Al), was carried out under field N manipulation (10-year N addition and subsequent cessation of N addition) in a typical, subtropical Masson pine (Pinus massoniana Lamb.) forest in southwest China. Results: Needle N and P concentrations were significantly elevated by N addition even after N addition ceased while needle N:P remained constant among treatments. Needle N:K and P:K were significantly elevated by N addition, but were no longer different from reference plots after N addition ceased. Masson pine's annual relative growth rate (RGR) was significantly decreased by N addition but the differences among treatments became insignificant among treatments after N addition ceased. Additionally, the annual RGR was negatively correlated with needle Al concentration. Needle Al exhibited a trend of first rising and then decreasing over the 14-year experiment. Conclusions: Under the background of high ambient N deposition and Ca deposition, chronic N addition does not aggravate P limitation but can result in K limitation in a N-saturated subtropical forest. With the recovery of subtropical forest from soil acidification and N saturation, the elevated needle N and P induced by N addition will persist for at least four years and most likely for much longer in the future, but the risk of K limitation is expected to diminish. [ABSTRACT FROM AUTHOR]

Elevated nitrogen (N) deposition and changes in reduced or oxidized component contribution greatly affect soil acidification. China has experienced a significant transformation of N deposition components from NHx to NOy over the past 40 years, but the effects of component transformation on soil acidification are poorly understood. Therefore, long‐term monitoring data and literature on N deposition, combined with the results of isotope experiments, were used to explore the contributions of different N forms on soil acidification in China's forests. Here, all processes related to NHx and NOy, including the transformation to NH4+ and NO3− and subsequent N cycling in the soil, were considered. We found that N‐induced soil acidification in 80% area of China's forests was dominated by NHx deposition, and the other areas (South China) were dominated by NOy deposition in 2010s. From 1980 to 2019, the average contribution of NHx was higher than that of NOy but the latter contribution continued to increase. Meanwhile, the results showed that soil acidification increased with the decrease of the ratio of NHx to NOy (RNHx/NOy), which is mainly because NOy is more easily leached in the form of NO3− than that of NHx under the influence of different plant preferences and soil retention rates, resulting in a higher net proton production of NOy. Our research has powerful implications for policymaking, provides a theoretical basis for formulating different N reduction policies in different regions, and points out that the synergistic effect of RNHx/NOy changes should be considered to alleviate soil acidification. Key Points: Most forest soil was dominated by the contribution of NHx deposition, causing soil acidificationThe contribution of NOy deposition to soil acidification increased continuously during 1980–2019Decreased ratio of NHx to NOy deposition has led to aggravated forest soil acidification [ABSTRACT FROM AUTHOR]

Elevated nitrogen (N) deposition may further acidify soils in tropical forests. Yet, we have limited evidence on this prediction and it remained unclear how this would affect fine root growth therein. Here, we report responses of fine root biomass, vitality, and chemistry, as well as related soil parameters to eight years of N additions in three tropical forests different in initial soil N status, with one primary forest being N-saturated, and another two younger forests (one secondary forest and one planted forest) less N-rich. Results showed that in the primary forest, fine root biomass decreased and fine root necromass increased following N addition, resulting in lower live fine root proportion (fine root vitality). Declining fine root vitality was associated with fine root Fe accumulation and soil acidification indicated by regression analysis. These alterations of fine root growth and chemistry co-occurred with soil pH decline, soil exchangeable Fe3+ mobilization, exchangeable Ca2+, and Mg2+ depletion after N treatments in the primary forest. In contrast, N addition only elevated fine root K, Al, and Fe content in the secondary forest. In the planted forest, moderate but significant decreases in soil pH, soil exchangeable K+, and Mg2+ were found after N treatment, with fine root biomass negatively correlated with soil exchangeable Al3+ and Al3+/(Ca2+ + Mg2+) ratio. Our results suggested that long-term N fertilization may negatively affect fine root growth, via severed soil acidification, Fe mobilization, and base cation depletion in highly acidified, N-saturated primary tropical forests. Initial forest N status, influenced by different land-use history, mediates N deposition effects on fine root growth. [ABSTRACT FROM AUTHOR]

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH ) and nitrate (NO ). Previous long-term studies indicate efficient turnover of atmogenic NH to NO in weathered, acidic soils of the subtropics, leading to excessive NO leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. NHNO, NHNO and N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (NO) were monitored for nine days. Incorporation of NO into soil organic N was negligible and 80% of the applied label was lost from the top soil (0-15 cm) primarily by leaching within 9 days. In contrast, NH was largely retained in soil organic N. However, instant production of NO in the NH treatment suggested active nitrification. In both the NH and N-glutamic acid treatments, the N enrichment in the NO pool exceeded that in the NH pool one day after N application, suggesting preferential nitrification of added NH with subsequent dilution of the NH pool and/or immobilization of NH followed by heterotrophic nitrification. The cumulative recovery of N in NO after 9 days ranged from 2.5 to 6.0% in the NO treatment, confirming the previously reported significant response of NO emission to N deposition. Source partitioning of NO demonstrated a measurable contribution of nitrification to NO emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO and NH ) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO in the soil and (ii) rapid immobilization-mineralization of NH followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and NO emission. [ABSTRACT FROM AUTHOR]

Chronically elevated nitrogen (N) deposition has led to severe nutrient imbalance in forest soils. Particularly in tropical and subtropical forest ecosystems, increasing N loading has aggravated phosphorus (P) limitation of biomass production, and has resulted in elevated emissions of nitrous oxide (N2O) and reduced uptake of methane (CH4), both of which are important greenhouse gases. Yet, the interactions of N and P and their effects on GHG emissions remain understudied. Here, we report N2O and CH4 emissions together with soil chemistry data for the a period of 18 months following P addition (79 kg P ha-1 yr-1, applied as NaH2PO4 powder) to a N-saturated, Masson pine-dominated forest at TieShanPing (TSP), Chongqing, SW China. We observed a significant decline both in NO3- concentrations in soil water (at 5- and 20-cm depths) and in N2O emissions, the latter by 3 kg N ha-1 yr-1. We hypothesize that enhanced N uptake by plants and soil microbes in response to P addition, results in less available NO3- for denitrification. By contrast to most other forest ecosystems, TSP is a net source of CH4. As for N2O, P addition significantly decreased CH4 emissions, turning the soil into a net sink. Based on our data and previous studies in South America and China, we believe that P addition relieves N-inhibition of CH4 oxidation. Within the 1.5 years after P addition, no significant increase of forest growth was observed at TSP, but we cannot exclude that understory vegetation increased. Our study suggests that P fertilization of acid forest soils could mitigate GHG emissions in addition to alleviate nutrient imbalances and reduce losses of nitrogen through NO3- leaching and N2O emission. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) in forest soil is of major importance for cation binding and acid buffering, but its characteristics may differ among soils under different tree species. We investigated acidity, cation exchange properties and Al bonding to SOM in stands of Scots pine, pedunculate oak, Norway spruce, European beech and common hornbeam in southern Poland. The content of total carbon (C t ) was by far the major contributor to total cation exchange capacity (CEC t ) even in loamy soils and a strong relationship between C t and CEC t was found. The slope of the regression of CEC t to C t increased in the order hornbeam ≈ oak < beech < spruce ≈ pine, suggesting that the number of negatively charged sites of SOM at any value in the acid pH range was smallest for hornbeam and oak, and largest for spruce and pine soils. This was supported by the apparent dissociation constant (pK app ) values of SOM, which were largest in soils under oak. The maximum values of Al saturation were similar between the stands. However, maximum Al bonding to SOM occurred at higher pH values in soils under pine and spruce than under oak. Therefore, at any value in the acid pH range, the SOM in pine soil has less Al complexed and more adsorbed H + than SOM from oak soils. Such differences in Al and H bonding are not only important for pH buffering and metal solubility controls, but also for stabilization of SOM via saturation of functional groups by Al and H. [ABSTRACT FROM AUTHOR]

Abstract: Dissolved aluminium (Al) in soils, mobilized by acid deposition, is considered a threat to forest health through hampering root growth and nutrient uptake. Since the end of the 1980s dissolved Al in forest soil water plays a key role in the assessment of critical loads of acid deposition. So far, most evidence for toxicity of dissolved Al in forest soil water is based on nutrient solution studies and pot experiments. Here, we present results from one of the few in situ ecosystem-scale forest manipulation experiments to study the effect of Al on mature forest trees. A plotwise addition of dilute AlCl3 was conducted during seven years in an even-aged spruce forest (Picea abies) in an area in Norway with low acid deposition. Soil solution concentrations of Al were increased to potentially toxic levels (up to 500μmolL−1) and base cation (Ca+Mg+K) to inorganic Al ratios in the soil solution in the root zone were mostly below 1 in the Al-addition treatments. In the control treatment (only water addition) Al concentrations did not exceed 15μmolL−1 and base cation to inorganic Al ratios were above 1. The toxic effects of Al on fine root growth and plant growth found in hydroponic studies and pot trials are not confirmed by this field manipulation. However, magnesium (Mg) contents in needles decreased significantly and persistently in plots with elevated Al concentrations, whereas the needle Ca content did not respond. The depletion of the Mg content in needles is suggested to be due to antagonistic effects of high Al concentrations at the root surface, consistent with observed reductions in Mg to Al ratio of inner bark. This study clearly supports a role for Al in critical load functions for forests as dissolved Al causes a decrease in uptake of Mg. However, other signs of reduced forest vitality were not observed. Soil base cation status may need to be included in risk evaluations of forest health under acid deposition. [Copyright &y& Elsevier]

Aluminium (Al) is a key element in critical load calculations for forest. Here, we argue for re-evaluating the importance of Al. Effects of two levels of enhanced Al concentrations and lowered Ca:Al ratios in the soil solution in a field manipulation experiment in a mature spruce stand (1996–1999) on tree vitality parameters were tested. In addition, Al solubility controls were tested. Various loads of Al were added to forest plots by means of an irrigation system. Potentially toxic Al concentrations and critical ratios of Ca to inorganic Al were established. The ratio of Ca to total Al was not a suitable indicator for unfavourable conditions for plant growth. No significant effects on crown condition, tree growth and fine root production were observed after three years of treatment. In 1999, foliar Mg content in the highest Al addition treatment had declined significantly. This agreed with the known response to Al stress of seedlings in nutrient solution experiments. No support was found for using the chemical criterion Ca:Al ratio in soil solution, foliar and root tissue as an indicator for forest damage due to acidification. Al solubility was considerably lower than implied by the assumption of equilibrium with gibbsite, particularly in the root zone. The gibbsite equilibrium is commonly used in critical load models. Substitution of the gibbsite equilibrium with an Al-organic matter complexation model to describe Al solubility in soil water may have large consequences for calculation of critical loads. The results indicate that critical load maps for forests should be reconsidered. [ABSTRACT FROM AUTHOR]

Denitrification characteristics of forest soils from eight headwater catchments in China were investigated in this study, along a climatic gradient from the tropics in the South to the temperate zones. Within each catchment, different landscape positions along hydrological flow paths were also considered, including well-drained soils on hill slopes and poorly drained soils in groundwater discharge zones. The results showed that instantaneous denitrification rates were much greater in soils from the northern sites than those from the southern sites (with the average of 110.0 and 25.4 nmol N g −1 dry soil h. − 1 , respectively). Large potentials for nitrous oxide (N 2 O) loss (evaluated as maximum N 2 O accumulation before it was reduced to dinitrogen (N 2 )) were observed in the six tropical and subtropical catchments, particularly in soils with high carbon (C) and nitrogen (N). Meanwhile high N 2 O / (N 2 O + N 2 ) stoichiometries were displayed in soils from these southern sites. Within catchments, soils from the groundwater discharge zones showed greater potential denitrification rates but smaller N 2 O / (N 2 O + N 2 ) ratios in comparison with those on the hill slopes, implying large N removal potentials of soils from the groundwater discharge zones. Furthermore, our findings suggest soil pH is the key controller for the potential denitrification rates and the N 2 O / (N 2 O + N 2 ) stoichiometries. Soil pH, C and N availability affect the potential for N 2 O loss synergistically. Our findings not only pinpoint the denitrification phenotypes of soils along the climatic gradient, but also confirm the small-scale variations within catchments which reflect the in situ habitat of the denitrifiers. These indicate the importance of discrimination related to different landscape positions when modeling N 2 O emissions and N removals from regional N loading. [ABSTRACT FROM AUTHOR]

Abstract: Liming of strongly acidified soil under a Masson pine (Pinus massoniana Lamb.) forest was studied through a seven-year field manipulation experiment at Tieshanping, Chongqing in Southwestern China. To distinguish between the individual effects of Ca2+ and Mg2+ addition, we separately applied calcite (CaCO3) and magnesite (MgCO3), rather than using dolomite [CaMg(CO3)2]. Both calcite and magnesite additions caused a significant increase in pH and a decrease in dissolved inorganic monomeric aluminium (Ali) concentration of soil water. Ecological recovery included increases of herb biomass (both treatments) and Mg content in Masson pine needles (magnesite treatment only). However, the growth rate of Masson pine did not increase under either treatment, possibly because of nutrient imbalance due to phosphorus (P) deficiency or limited observation period. In China, acid deposition in forest ecosystems commonly coincides with large inputs of atmogenic Ca2+, both enhancing Mg2+ leaching. Calcite addition may further decrease the Mg2+ availability in soil water, thereby exacerbating Mg2+ deficiency in the acidified forest soils of southern and southwestern China. The effect of anthropogenic acidification of naturally acid forest soils on P availability needs further study. [Copyright &y& Elsevier]