Savanna regions are increasingly developed for agriculture to support population growth, food demand and export economies. This is driving interest in the conversion of natural savanna to cattle grazed pastures or horticultural crops in Northern Australia. Savanna clearing leads to aboveground carbon (C) loss but impacts below-ground are less certain as studies have focused on high rainfall regions, low reactive clay soils and shallow depths of 0.3 m or less.

To examine impact of land use change (LUC) in Northern Australia, we sampled the upper 1.0 m soil profile of: 1) savanna woodlands, 2) old and young C4 perennial cattle grazed pastures and 3) old and young, melon-sorghum rotation fields. Soil C concentrations and soil bulk density in the upper 0.3 m were significantly greater in cattle grazed pastures than savanna woodland, such that savanna to pasture LUC significantly increased soil C density from 30.2 ± 1.5–43.4 ± 3.0 Mg C ha^-1 over 28 years. Soil bulk density in the upper 0.7 m of savanna woodland was significantly less than that in the melon-sorghum rotations. In contrast, soil C concentrations below 0.5 m were significantly greater in the savanna woodland than in the 3 y.o. melon-sorghum rotation fields. There was no significant difference in soil C density between savanna woodland and 12 y.o. melon-sorghum rotations. This may relate to the beneficial effects of ploughing in sorghum as a green manure, or simply that the decrease in soil C concentration was offset by increased soil C density in the upper layers of soil where most C is located. In contrast, conversion of savanna to cattle grazed pasture sites is equivalent to a soil sequestration rate of 0.34 Mg C ha^-1 y^-1, approximately 50% of the observed net ecosystem productivity estimated using eddy covariance methods. Changing the depth to which soil C density was estimated changed significant differences amongst land-use systems. Whereas, estimating soil C density using a ‘fixed depth’ or ‘equivalent mass’ method did not change the statistical significance.

There is an increasing focus on the potential of soil carbon sequestration especially in north Australia and understanding potential change and impact of accounting methods is essential for meaningful policy development. Policy settings to maintain, or improve, soil C storage and soil health may contradict agricultural development goals.

稀树草原地区越来越多地发展农业，以支持人口增长、粮食需求和出口经济。这引起了人们对澳大利亚北部将天然稀树草原转变为放牧牧场或园艺作物的兴趣。稀树草原清理导致地上碳 (C) 损失，但地下影响不太确定，因为研究集中在高降雨区、低活性粘土和 0.3 m 或更小的浅层深度。

为了检查澳大利亚北部土地利用变化 (LUC) 的影响，我们对以下 1.0 m 的土壤剖面进行了采样：1) 稀树草原林地，2) C4 多年生牛放牧牧场和 3) 老幼，瓜 - 高粱轮作领域。牛放牧牧场上部 0.3 m 的土壤 C 浓度和土壤容重显着高于稀树草原林地，因此稀树草原到牧场 LUC 显着增加了土壤 C 密度，从 30.2 ± 1.5–43.4 ± 3.0 Mg C ha ^-1超过 28 年。稀树草原林地上部 0.7 m 的土壤容重显着低于甜瓜-高粱轮作。相比之下，稀树草原林地中低于 0.5 m 的土壤 C 浓度显着高于 3 年甜瓜 - 高粱轮作田。稀树草原林地和 12 年甜瓜-高粱轮作之间的土壤碳密度没有显着差异。这可能与高粱作为绿肥耕作的有益影响有关，或者仅仅是土壤碳浓度的下降被大多数碳所在的土壤上层土壤碳密度的增加所抵消。相比之下，将稀树草原转化为牛放牧的牧场相当于土壤封存率为 0.34 Mg C ha ^-1 y ^-1, 大约 50% 的观察到的净生态系统生产力是使用涡流协方差方法估计的。改变估计土壤碳密度的深度会改变土地利用系统之间的显着差异。然而，使用“固定深度”或“等效质量”方法估算土壤碳密度并没有改变统计显着性。

人们越来越关注土壤碳固存的潜力，尤其是在澳大利亚北部，了解核算方法的潜在变化和影响对于制定有意义的政策至关重要。维持或改善土壤碳储存和土壤健康的政策设置可能与农业发展目标相矛盾。