Data from: Soil nitrogen-cycling responses to conversion of lowland forests to oil palm and rubber plantations in Sumatra, Indonesia

Rapid deforestation in Sumatra, Indonesia is presently occurring due to
the expansion of palm oil and rubber production, fueled by an increasing
global demand. Our study aimed to assess changes in soil-N cycling rates
with conversion of forest to oil palm (Elaeis guineensis) and rubber
(Hevea brasiliensis) plantations. In Jambi Province, Sumatra, Indonesia,
we selected two soil landscapes – loam and clay Acrisol soils – each with
four land-use types: lowland forest and forest with regenerating rubber
(hereafter, “jungle rubber”) as reference land uses, and rubber and oil
palm as converted land uses. Gross soil-N cycling rates were measured
using the 15N pool dilution technique with in-situ incubation of soil
cores. In the loam Acrisol soil, where fertility was low, microbial
biomass, gross N mineralization and NH4+ immobilization were also low and
no significant changes were detected with land-use conversion. The clay
Acrisol soil which had higher initial fertility based on the reference
land uses (i.e. higher pH, organic C, total N, effective cation exchange
capacity (ECEC) and base saturation) (P≤0.05–0.09) had larger microbial
biomass and NH4+ transformation rates (P≤0.05) compared to the loam
Acrisol soil. Conversion of forest and jungle rubber to rubber and oil
palm in the clay Acrisol soil decreased soil fertility which, in turn,
reduced microbial biomass and consequently decreased NH4+ transformation
rates (P≤0.05–0.09). This was further attested by the correlation of gross
N mineralization and microbial biomass N with ECEC, organic C, total N
(R=0.51–0. 76; P≤0.05) and C:N ratio (R=-0.71 – -0.75, P≤0.05). Our
findings suggest that the larger the initial soil fertility and N
availability, the larger the reductions upon land-use conversion. Because
soil N availability was dependent on microbial biomass, management
practices in converted oil palm and rubber plantations should focus on
enriching microbial biomass.