Changes in above- and below-ground nitrogen stocks and allocations following the conversion of farmland to forest in rocky desertification regions

Afforestation of degraded land is one of the principal strategies for preventing soil erosion and promoting ecosystem recovery in fragile regions, especially in rocky desertification areas. In China, millions of hectares of farmland have been converted into forest in order to arrest and reverse rocky desertification under the Grain for Green Program (GGP). This study evaluated implications of land-use change from annual maize cultivation to perennial Zanthoxylum bungeanum plantations (1-, 4-, 7-, and 10-year-old) in the karst region of Guizhou province, southwest China. The study analyses the variations of biomass and nitrogen (N) storages as well as N distributions in biomass components and soil depths. Results showed that the N content in components of Z. bungeanum ranged from 0.31% to 3.24% with a mean value of 1.75%, which was lower than that of maize (2.13%) in the same region. The biomass N storage measured for the maize cropland was 210.59 kg ha⿿1, while this value increased linearly with stand ages for the four Z. bungeanum plantations (0.94, 108.31, 212.20, and 262.12 kg ha⿿1, respectively). The average amount of soil N storage in the Z. bungeanum plantations (9.33 t ha⿿1) was significantly lower than in the adjacent intensively managed maize cropland (10.04 t ha⿿1). This is mainly due to long-term organic and inorganic fertilizer inputs in the farmland stage. Total ecosystem N storage averaged 10.25 t ha⿿1 in the maize cropland, and 9.38, 9.82, 9.05, and 9.67 t ha⿿1 in the 1-, 4-, 7- and 10-year-old Z. bungeanum plantations, respectively. Soils accounted for 97% of total ecosystem N storage in both land-use systems. This study suggests that the reduction of surface soil disturbance during plantation management practices plays a crucial role in improving the N storage. Data of annual plantation area and biomass N accumulation rates under the GGP indicate that Guizhou province was a net N sink with 2.35 ÿ 108 kg N in 2010, corresponding to 41.45% of N (NOX-N and NH3-N) emissions in that year. Besides increasing N sequestration over time (as these forest mature), the large-scale plantations of Z. bungeanum have the potential to restore severely degraded soils in the karst region of SW China.