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Soil Biology and Biochemistry

Soil Nitrogen Cycling following Montane Forest Conversion in Central Sulawesi, Indonesia

Institute of Soil Science and Forest Nutrition, Univ. of Goettingen, Buesgenweg 2, Goettingen 37077, Germany

^* Corresponding author (mcorre{at}gwdg.de)

The lower montane forest zone of Indonesia is undergoing rapid conversion of indigenous forests to agriculture. In this tropical region, however, the effects of forest conversion on soil N processes have not been investigated. Corn (Zea mays L.) and cacao (Theobroma cacao L.)–coffee (Coffea canephora Pierre ex Froehner) agroforestry are the main land use types in cleared lower montane forests in Central Sulawesi, Indonesia. Our main objective was to compare the soil N dynamics under agroforest systems and corn cultivation with indigenous forest. We measured the gross rates of N transformation processes using ¹⁵N pool dilution. The agroforest systems and indigenous forests had higher gross N mineralization rates and faster turnover rates of NH₄⁺ and microbial N pools than the long-term cultivated corn sites. Faster soil N turnover rates in agroforest systems suggest a more dynamic soil N cycling. Leguminous shade trees, which are important components of these agroforest systems, may have influenced the fast microbial N cycling through release of N-rich root exudates and plant residues. Our results show that compared with corn, agroforestry is a better option in terms of sustainability in the N-supplying capacity of the soil. In addition, we measured higher ¹⁵NH₄⁺ recoveries than ¹⁵NO₃^– recoveries after 15 min of ¹⁵N addition in all our sites. Our measured rates of gross nitrification were very low to negligible, due to rapid disappearance of added ¹⁵NO₃^–. Such fast reaction of NO₃^– warrants further investigation, especially in tropical areas where ¹⁵N studies are very few.

Abbreviations: ISSFN, Institute of Soil Science and Forest Nutrition • MBN, microbial biomass nitrogen • MRT, mean residence time