Abstract. Oil palm and rubber plantations cover large areas of former rainforest in Sumatra, Indonesia, supplying the global demand for these crops. Although forest conversion is known to influence soil nitrous oxide (N2O) and nitric oxide (NO) fluxes, measurements from oil palm and rubber plantations are scarce (for N2O) or nonexistent (for NO). Our study aimed to (1) quantify changes in soil-atmosphere fluxes of N oxides with forest conversion to rubber and oil palm plantations, and (2) determine their controlling factors. In Jambi, Sumatra, we selected two landscapes that mainly differed in texture but both on heavily weathered soils: loam and clay Acrisol soils. Within each landscape, we investigated lowland forest, rubber trees interspersed in secondary forest (termed as jungle rubber), both as reference land uses, and smallholder rubber and oil palm plantations, as converted land uses. Each land use had four replicate plots within each landscape. Soil N2O fluxes were measured monthly from December 2012 to December 2013, and soil NO fluxes were measured four times between March and September 2013. In the loam Acrisol landscape, we also conducted weekly to bi-weekly soil N2O flux measurements from July 2014 to July 2015 in a large-scale oil palm plantation with four replicate plots for comparison with smallholder oil palm plantations. Land-use conversion to smallholder plantations had no effect on soil N-oxide fluxes (P = 0.58 to 0.76) due to the generally low soil N availability in the reference land uses that further decreased with land-use conversion. Over one-year measurements, the temporal patterns of soil N-oxide fluxes were influenced by soil mineral N and water contents. Across landscapes, annual soil N2O emissions were controlled by gross nitrification and sand content, which also suggest the influence of soil N and water availability. Soil N2O fluxes (µg N m−2 h−1) were: 7 ± 2 to 14 ± 7 (reference land uses), 6 ± 3 to 9 ± 2 (rubber), 12 ± 3 to 12 ± 6 (smallholder oil palm), and 42 ± 24 (large-scale oil palm). Soil NO fluxes (µg N m−2 h−1) were: −0.6 ± 0.7 to 5.7 ± 5.8 (reference land uses), −1.2 ± 0.5 to −1.0 ± 0.2 (rubber) and −0.2 ± 1.2 to 0.7 ± 0.7 (smallholder oil palm). The low N fertilizer application in smallholder oil palm plantations (commonly 48 to 88 kg N ha−1 yr−1) resulted in N-oxide losses of only 0.2–0.7 % of the applied N. To improve estimate of soil N-oxide fluxes from oil palm plantations in this region, studies should focus on large-scale plantations (which usually have two to four times higher N fertilization rates than smallholders) with frequent measurements following fertilizer application.
Soil nitrogen oxide fluxes from lowland forests converted smallholder rubber and oil palm plantations in Sumatra, Indonesia