Abstract. Integrated crop–pasture rotational systems can store larger soil organic carbon (SOC) stocks in the topsoil (0–20 cm) than continuous grain cropping. The aim of this study was to identify if the main determinant for this difference may be the avoidance of old C losses in integrated systems or the higher rate of new C incorporation associated with higher C input rates. We analyzed the temporal changes of 0–20 cm SOC stocks in two agricultural treatments of different intensity (continuous annual grain cropping and crop–pasture rotational system) in a 60-year experiment in Colonia, Uruguay. We incorporated this information into a process of building and parameterizing SOC compartmental dynamical models, including data from SOC physical fractionation (particulate organic matter, POM > 53 µm > mineral-associated organic matter, MAOM), radiocarbon in bulk soil, and CO2 incubation efflux. This modeling process provided information about C outflow rates from pools of different stability, C stabilization dynamics, and the age distribution and transit times of C. The differences between the two agricultural systems were mainly determined by the dynamics of the slow-cycling pool (∼MAOM). The outflow rate from this compartment was between 3.68 and 5.19 times higher in continuous cropping than in the integrated system, varying according to the historical period of the experiment considered. The avoidance of old C losses in the integrated crop–pasture rotational system resulted in a mean age of the slow-cycling pool (∼MAOM) of over 600 years, with only 8.8 % of the C in this compartment incorporated during the experiment period (after 1963) and more than 85 % older than 100 years old in this agricultural system. Moreover, half of the C inputs to both agricultural systems leave the soil in approximately 1 year due to high decomposition rates of the fast-cycling pool (∼POM). Our results show that the high capacity to preserve old C of integrated crop–pasture systems is the key for SOC preservation of this sustainable intensification strategy, while their high capacity to incorporate new C into the soil may play a second role. Maintaining high rates of C inputs and relatively high stocks of labile C appear to be a prerequisite for maintaining low outflow rates of the MAOM pool.

中文翻译：

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一项已有 60 年历史的实验评估了作物-牧场综合系统保存旧土壤碳的高能力


摘要。与连续种植谷物相比，综合作物-牧草轮作系统可以在表土（0-20cm）中储存更多的土壤有机碳（SOC）。本研究的目的是确定这种差异的主要决定因素是否可能是避免集成系统中旧的碳损失，或者与较高的碳输入率相关的较高的新碳掺入率。我们在乌拉圭科洛尼亚进行了一项为期 60 年的实验，分析了两种不同强度的农业处理（粮食连作和作物-牧草轮作制度）下 0-20cm SOC 储量的时间变化。我们将这些信息纳入构建和参数化 SOC 分区动力学模型的过程中，包括来自 SOC 物理分馏（颗粒有机物，POM > 53 μm > 矿物相关有机物，MAOM）、散装土壤中的放射性碳和 CO2 孵化流出的数据。该建模过程提供了有关不同稳定性池的碳流出速率、碳稳定动态以及碳的年龄分布和运输时间的信息。两种农业系统之间的差异主要由慢循环池的动态决定（∼毛姆）。连作中该隔间的流出速率比综合系统高 3.68 至 5.19 倍，具体取决于所考虑的实验的历史时期。作物-牧草综合轮作系统中避免旧碳损失导致慢速循环池 (∼MAOM) 的平均年龄超过 600 年，在实验期间，该区划中仅吸收了 8.8% 的碳（ 1963 年之后），并且在这个农业系统中超过 85% 的年龄超过 100 岁。 此外，由于快速循环池（∼POM）的高分解率，两个农业系统的一半碳输入在大约一年内离开土壤。我们的研究结果表明，作物-牧场综合系统保留旧碳的高能力是这种可持续集约化策略保持SOC的关键，而它们将新碳融入土壤的高能力可能发挥第二个作用。维持高碳输入率和相对较高的活性碳库存似乎是维持 MAOM 池低流出率的先决条件。