No abstract

Nutrient deposition to tropical forests is increasing, which could affect soil fluxes of nitrous oxide (N 2 O), a powerful greenhouse gas. We assessed the effects of 35-56 months of moderate nitrogen (N) and phosphorus (P) additions on soil N 2 O fluxes and net soil N-cycling rates, and quantified the relative contributions of nitrification and denitrification to N 2 O fluxes. In 2008, a nutrient manipulation experiment was established along an elevation gradient (1000, 2000, and 3000 m) of montane forests in southern Ecuador. Treatments included control, N, P, and N+P addition (with additions of 50 kg N ha −1 yr −1 and 10 kg P ha −1 yr −1 ). Nitrous oxide fluxes were measured using static, vented chambers and N cycling was determined using the buried bag method. Measurements showed that denitrification was the main N 2 O source at all elevations, but that annual N 2 O emissions from control plots were low, and decreased along the elevation gradient (0.57 ± 0.26-0.05 1 1 ±0.04 kg N 2 O-N ha − yr − ). We attributed the low fluxes to our sites' conservative soil N cycling as well as gaseous N losses possibly being dominated by N 2 . Contrary to the first 21 months of the experiment, N addition did not affect N 2 O fluxes during the 35-56 month period, possibly due to low soil moisture contents during this time. With P addition, N 2 O fluxes and mineral N concentrations decreased during Months 35-56, presumably because plant P limitations were alleviated, increasing plant N uptake. Nitrogen plus phosphorus addition showed similar trends to N addition, but less pronounced given the counteracting effects of P addition. The combined results from this study (Months 1-21 and 35-56) showed that effects of N and P addition on soil N 2 O fluxes were not linear with time of exposure, highlighting the importance of long-term studies.

Abstract. Tropical lowland forest soils are significant sources and sinks of trace gases. In order to model soil trace gas flux for future climate scenarios, it is necessary to be able to predict changes in soil trace gas fluxes along natural gradients of soil fertility and climatic characteristics. We quantified trace gas fluxes in lowland forest soils at five locations in Panama, which encompassed orthogonal precipitation and soil fertility gradients. Soil trace gas fluxes were measured monthly for 1 (NO) or 2 (CO 2 , CH 4 , N 2 O) years (2010-2012) using vented dynamic (for NO only) or static chambers with permanent bases. Across the five sites, annual fluxes ranged from 8.0 to 10.2 Mg CO 2 -C, −2.0 to −0.3 kg CH 4 -C, 0.4 to 1.3 kg N 2 O-N and −0.82 to −0.03 kg NO-N ha −1 yr −1 . Soil CO 2 emissions did not differ across sites, but they did exhibit clear seasonal differences and a parabolic pattern with soil moisture across sites. All sites were CH 4 sinks; within-site fluxes were largely controlled by soil moisture, whereas fluxes across sites were positively correlated with an integrated index of soil fertility. Soil N 2 O fluxes were low throughout the measurement years, but the highest emissions occurred at a mid-precipitation site with high soil N availability. Net negative NO fluxes at the soil surface occurred at all sites, with the most negative fluxes at the low-precipitation site closest to Panama City; this was likely due to high ambient NO concentrations from anthropogenic sources. Our study highlights the importance of both shortterm (climatic) and long-term (soil and site characteristics) factors in predicting soil trace gas fluxes.

Abstract. Conversion of forest to rubber and oil palm plantations is widespread in Sumatra, Indonesia, and it is largely unknown how such land-use conversion affects nutrient leaching losses. Our study aimed to quantify nutrient leaching and nutrient retention efficiency in the soil after land-use conversion to smallholder rubber and oil palm plantations. In Jambi province, Indonesia, we selected two landscapes on highly weathered Acrisol soils that mainly differed in texture: loam and clay. Within each soil type, we compared two reference land uses, lowland forest and jungle rubber (defined as rubber trees interspersed in secondary forest), with two converted land uses: smallholder rubber and oil palm plantations. Within each soil type, the first three land uses were represented by 4 replicate sites and the oil palm by three sites, totaling 30 sites. We measured leaching losses using suction cup lysimeters sampled biweekly to monthly from February to December 2013. Forests and jungle rubber had low solute concentrations in drainage water, suggesting low internal inputs of rock-derived nutrients and efficient internal cycling of nutrients. These reference land uses on the clay Acrisol soils had lower leaching of dissolved N and base cations (P= 0.01–0.06) and higher N and base cation retention efficiency (P < 0.01–0.07) than those on the loam Acrisols. In the converted land uses, particularly on the loam Acrisol, the fertilized area of oil palm plantations showed higher leaching of dissolved N, organic C, and base cations (P < 0.01–0.08) and lower N and base cation retention efficiency compared to all the other land uses (P < 0.01–0.06). The unfertilized rubber plantations, particularly on the loam Acrisol, showed lower leaching of dissolved P (P=0.08) and organic C (P < 0.01) compared to forest or jungle rubber, reflecting decreases in soil P stocks and C inputs to the soil. Our results suggest that land-use conversion to rubber and oil palm causes disruption of initially efficient nutrient cycling, which decreases nutrient availability. Over time, smallholders will likely be increasingly reliant on fertilization, with the risk of diminishing water quality due to increased nutrient leaching. Thus, there is a need to develop management practices to minimize leaching while sustaining productivity.

Although the canopy can play an important role in forest nutrient cycles, canopy-based processes are often overlooked in studies on nutrient deposition. In areas of nitrogen (N) and phosphorus (P) deposition, canopy soils may retain a significant proportion of atmospheric inputs, and also receive indirect enrichment through root uptake followed by throughfall or recycling of plant litter in the canopy. We measured net and gross rates of N cycling in canopy soils of tropical montane forests along an elevation gradient and assessed indirect effects of elevated nutrient inputs to the forest floor. Net N cycling rates were measured using the buried bag method. Gross N cycling rates were measured using (15) N pool dilution techniques. Measurements took place in the field, in the wet and dry season, using intact cores of canopy soil from three elevations (1000, 2000 and 3000 m). The forest floor had been fertilized biannually with moderate amounts of N and P for 4 years; treatments included control, N, P, and N + P. In control plots, gross rates of NH4 (+) transformations decreased with increasing elevation; gross rates of NO3 (-) transformations did not exhibit a clear elevation trend, but were significantly affected by season. Nutrient-addition effects were different at each elevation, but combined N + P generally increased N cycling rates at all elevations. Results showed that canopy soils could be a significant N source for epiphytes as well as contributing up to 23% of total (canopy + forest floor) mineral N production in our forests. In contrast to theories that canopy soils are decoupled from nutrient cycling in forest floor soil, N cycling in our canopy soils was sensitive to slight changes in forest floor nutrient availability. Long-term atmospheric N and P deposition may lead to increased N cycling, but also increased mineral N losses from the canopy soil system.