Modest Gaseous Nitrogen Losses Point to Conservative Nitrogen Cycling in a Lowland Tropical Forest Watershed

Assessing the changes in river water quality across a land-use change (forest to oil palm plantation) in peninsular Malaysia using the stable isotopes of water and nitrate

Land conversion from natural forests to plantations (e.g., oil palm) in Southeast Asia is one of the most intensive land-use changes occurring worldwide. To clarify the effects of oil palm plantations on water quality, we conducted multipoint river and stream water sampling in peninsular Malaysia at the end of the rainy season over a 3-year period (2013-2015). We measured the major dissolved ions and stable isotope ratios of water (δ²H-H₂O and δ¹⁸O-H₂O) and nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in water from the upper streams in mountainous forests to the midstream areas of two major rivers in peninsular Malaysia. The electrical conductivity increased, and the d-excess value (as an index of the degree of evaporation) decreased with increasing distance from the headwaters, suggesting the effect of evaporative enrichment and the addition of pollutants. We separated the sampling points into four groups (G1-G4) through cluster analysis of the water quality data. From the land use/land cover (LULC) classification maps developed from satellite images and local information, we found that G1 and G2 mainly consisted of sampling points in forested areas, while G3 and G4 were located in oil-palm-affected areas. The concentrations of major ions were higher in the oil palm areas, indicating the effects of fertilizer and limestone (i.e., pH adjustment) applications. The dissolved inorganic nitrogen concentration did not differ among the groups, but the dissolved organic carbon, total dissolved nitrogen, and δ¹⁵N-NO₃^- were higher in the oil palm area than in the forested area. Although the nitrogen concentration was low, even in the oil palm area, the significantly higher δ¹⁵N-NO₃^- in the oil palm area indicated substantial denitrification. This implies that denitrification contributed to the lowering of the NO₃^- concentration in rivers in the oil palm area, in addition to nutrient uptake by oil palm trees.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China

Background

Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO₃ ^- loss potential following secondary succession are not well understood in karst ecosystems.

Methods

In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and ¹⁵N natural abundance (δ ¹⁵N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ ¹⁵N compositions of soil organic nitrogen (SON) in aggregates and soil to plant ¹⁵N enrichment factor (EF = δ ¹⁵N_leaf -δ ¹⁵N_soil), combined with soil inorganic N contents and δ ¹⁵N compositions were used to indicate the alterations of soil N availability and NO₃ ^-loss potential following secondary succession.

Results

Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ ¹⁵N values of SON also significantly decreased, mainly affected by plant δ ¹⁵N composition and N mineralization. SON content in macro-aggregates and soil NH₄ ⁺ content significantly increased while δ ¹⁵N values of NH₄ ⁺ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ ¹⁵N values, the EF values, and the (NO₃ ^--N)/(NH₄ ⁺-N) ratio gradually decreased, indicating reduced NO₃ ^- loss following secondary succession.

Conclusions

Soil N availability improves and NO₃ ^- leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Stable nitrogen and carbon isotope compositions in plant-soil systems under different land-use types in a red soil region, Southeast China

Background

Stable N isotope compositions in plant-soil systems have been widely used to indicate soil N transformation and translocation processes in ecosystems. However, soil N processes and nitrate ( NO 3 - ) loss potential under different land-use types are short of systematic comparison in the red soil region of Southeast China.

Methods

In the present study, the stable N and C isotope compositions (δ ¹⁵N and δ ¹³C) of soil and leaf were analyzed to indicate soil N transformation processes, and the soil to plant ¹⁵N enrichment factor (EF) was used to compare soil NO 3 - loss potential under different land-use types, including an abandoned agricultural land, a natural pure forest without understory, and a natural pure forest with a simple understory.

Results

The foliar δ ¹⁵N value (-0.8‰) in the abandoned agricultural land was greater than those of the forest lands (ranged from -2.2‰ to -10.8‰). In the abandoned agricultural land, δ ¹⁵N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ ¹³C values of soil organic carbon (SOC) decreased from -22.7‰ to -25.9‰ with increasing soil depth from 0-70 cm, mainly resulting from SON mineralization, soil organic matter (SOM) decomposition, and C₄ plant input. In the soils below 70 cm depth, δ ¹⁵N values of SON (mean 4.9‰) were likely affected by microbial assimilation of ¹⁵N-depleted NO 3 - . The variations in δ ¹⁵N values of soil profiles under the two forests were similar, but the EF values were significant different between the pure forest with a simple understory (-10.0‰) and the forest without understory (-5.5‰).

Conclusions

These results suggest that soil to plant ¹⁵N enrichment factor have a great promise to compare soil NO 3 - loss potential among different ecosystems.

Low N2O and variable CH4 fluxes from tropical forest soils of the Congo Basin

Globally, tropical forests are assumed to be an important source of atmospheric nitrous oxide (N₂O) and sink for methane (CH₄). Yet, although the Congo Basin comprises the second largest tropical forest and is considered the most pristine large basin left on Earth, in situ N₂O and CH₄ flux measurements are scarce. Here, we provide multi-year data derived from on-ground soil flux (n = 1558) and riverine dissolved gas concentration (n = 332) measurements spanning montane, swamp, and lowland forests. Each forest type core monitoring site was sampled at least for one hydrological year between 2016 - 2020 at a frequency of 7-14 days. We estimate a terrestrial CH₄ uptake (in kg CH₄-C ha⁻¹ yr⁻¹) for montane (−4.28) and lowland forests (−3.52) and a massive CH₄ release from swamp forests (non-inundated 2.68; inundated 341). All investigated forest types were a N₂O source (except for inundated swamp forest) with 0.93, 1.56, 3.5, and −0.19 kg N₂O-N ha⁻¹ yr⁻¹ for montane, lowland, non-inundated swamp, and inundated swamp forests, respectively.

The Congo Basin is home to the second largest stretch of continuous tropical forest, but the magnitude of greenhouse fluxes are poorly understood. Here the authors analyze gas samples and find the region is not actually a hotspot of N2O emissions.

In-depth analysis of N2O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis

Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (N₂O) or elemental nitrogen (N₂). The release of N₂O is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of N₂O; however, despite being the world's second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating N₂O fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual N₂O fluxes in the Congo Basin's lowland forests (0.97 ± 0.53 kg N ha^-1 year^-1) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha^-1 year^-1). Measurements of soil pore air N₂O isotope data at multiple depths suggests that a microbial reduction of N₂O to N₂ within the soil may account for the observed low surface N₂O fluxes and low soil pore N₂O concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (N₂O, N₂) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N₂ via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.

Light fuels while nitrogen suppresses symbiotic nitrogen fixation hotspots in neotropical canopy gap seedlings

Mature neotropical lowland forests have relatively lower symbiotic nitrogen fixation (SNF) rates compared with secondary forests. Canopy gap formation may create transient SNF hotspots in mature forests that increase overall SNF rates in these ecosystems, as canopy gaps are pervasive across the landscape and increasing in frequency. However, what environmental conditions are driving SNF upregulation in canopy gaps is unknown. In a field experiment to test these potential environmental controls on SNF, we grew 540 neotropical nitrogen-fixing legume seedlings (Pentaclethra macroloba, Zygia longifolia, and Stryphnodendron microstachyum) under manipulated light and soil nitrogen availability in canopy gaps and intact forests at La Selva Biological Station, Costa Rica. Seedling biomass, nodule biomass, and SNF (g N seedling^-1  h^-1 ) were 4-, 17- and 42-fold higher, respectively, in canopy gaps than in the intact forest. Nitrogen additions decreased SNF, but light had a stronger positive effect. Upregulation of SNF in canopy gaps was driven by increased plant growth and not a disproportionate increased SNF allocation. These data provide evidence that canopy gap SNF hotspots are driven, in part, by light availability, demonstrating a potential driver of SNF spatial heterogeneity. This further suggests that canopy gap dynamics are important for understanding the biogeochemistry of neotropical forests.

Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi-site dual nitrate isotopes

Increasing nitrogen (N) deposition in subtropical forests in south China causes N saturation, associated with significant nitrate (NO₃^- ) leaching. Strong N attenuation may occur in groundwater discharge zones hydrologically connected to well-drained hillslopes, as has been shown for the subtropical headwater catchment "TieShanPing", where dual NO₃^- isotopes indicated that groundwater discharge zones act as an important N sink and hotspot for denitrification. Here, we present a regional study reporting inorganic N fluxes over two years together with dual NO₃^- isotope signatures obtained in two summer campaigns from seven forested catchments in China, representing a gradient in climate and atmospheric N input. In all catchments, fluxes of dissolved inorganic N indicated efficient conversion of NH₄⁺ to NO₃^- on well-drained hillslopes, and subsequent interflow of NO₃^- over the argic B-horizons to groundwater discharge zones. Depletion of ¹⁵ N- and ¹⁸ O-NO₃^- on hillslopes suggested nitrification as the main source of NO₃^- . In all catchments, except one of the northern sites, which had low N deposition rates, NO₃^- attenuation by denitrification occurred in groundwater discharge zones, as indicated by simultaneous ¹⁵ N and ¹⁸ O enrichment in residual NO₃^- . By contrast to the southern sites, the northern catchments lack continuous and well-developed groundwater discharge zones, explaining less efficient N removal. Using a model based on ¹⁵ NO₃^- signatures, we estimated denitrification fluxes from 2.4 to 21.7 kg N ha^-1 year^-1 for the southern sites, accounting for more than half of the observed N removal. Across the southern catchments, estimated denitrification scaled proportionally with N deposition. Together, this indicates that N removal by denitrification is an important component of the N budget of southern Chinese forests and that natural NO₃^- attenuation may increase with increasing N input, thus partly counteracting further aggravation of N contamination of surface waters in the region.

Nitrogen cycling during secondary succession in Atlantic Forest of Bahia, Brazil

Carbon accumulation in tropical secondary forests may be limited in part by nitrogen (N) availability, but changes in N during tropical forest succession have rarely been quantified. We explored N cycle dynamics across a chronosequence of secondary tropical forests in the Mata Atlântica of Bahia, Brazil in order to understand how quickly the N cycle recuperates. We hypothesized that N fixation would decline over the course of succession as N availability and N gaseous losses increased. We measured N fixation, KCl-extractable N, net mineralization and nitrification, resin-strip sorbed N, gaseous N emissions and the soil δ15N in stands that were 20, 35, 50, and > 50 years old. Contrary to our initial hypothesis, we found no significant differences between stand ages in any measured variable. Our findings suggest that secondary forests in this region of the Atlantic forest reached pre-disturbance N cycling dynamics after just 20 years of succession. This result contrasts with previous study in the Amazon, where the N cycle recovered slowly after abandonment from pasture reaching pre-disturbance N cycling levels after ~50 years of succession. Our results suggest the pace of the N cycle, and perhaps tropical secondary forest, recovery, may vary regionally.