Agricultural practices such as annual crop production, land use change and grazing on marginal lands lead to a loss of soil carbon (C) stock. But soil C losses are not universal in agricultural systems and modest soil C gains can occur when constraints such as a lack of water are removed. To characterize this we used a meta-analysis of published data focused on semi-arid regions, where irrigation is required for crop production. We showed that soil C stocks declined under cereals, cotton, maize and non-woody horticultural crops when compared to native unirrigated adjacent grassland or shrubland. By contrast, cultivation of irrigated, woody perennial crops generally leads to an accumulation of soil C. Identifying the mechanisms by which C is retained in the soils beneath woody perennial crops, and any limits to C accumulation, was the main goal of this study. A mechanistic understanding of soil organic C content accumulation, upon land use change, can be gained by dividing soil into particulate organic matter (POM) and mineral associated organic matter (MAOM). Here, we analyzed the C and natural abundance ¹³C concentrations in POM and MAOM fractions in soils from eight apple orchards and eight vineyards irrigated using a dripline, and eight apple and eight cherry orchards irrigated with micro-spray. Samples were also taken from eight native grassland areas adjacent to the agricultural sites for comparison. Several decades of woody crop production doubled the average soil C concentration in comparison to the native sites, from to 10.1 ± 1.48 g C kg⁻¹ to 20.1 ± 0.96 g C kg⁻¹ over a depth of 0–15 cm. Most of the C was associated with POM, which increased in concentration from 7.9 ± 1.19 g C kg⁻¹ to 14.2 ± 0.79 g C kg⁻¹ in 0–15 cm soils, an increase of 80%. This was crop dependent, being highest in the cherry orchards and lowest in the vineyard soils. Although holding less C, the MAOM concentration increased by 166%, changing from 2.22 ± 0.33 to 5.91 ± 0.62 g C kg⁻¹; no differences existed between crops but the MAOM C concentration appeared to be constrained to a maximum value of ~ 12 g C kg⁻¹. MAOM and POM had markedly different δ¹³C values: MAOM was more enriched, indicative of greater microbial processing, whereas POM had a lower δ¹³C value consistent with the dominant standing vegetation. δ¹³C values were more depleted in both fractions at the agricultural sites compared to the native sites, indicating the accumulation of greater amounts of less processed C at the agricultural sites, due to higher C inputs. We conclude the soils in this region respond to irrigated perennial woody crop production by retaining C within both POM and MAOM fractions. Accumulation of C as MAOM is constrained by the minerology of the soils in this region, but is unconstrained for POM which dominates the soil C content, and is potentially vulnerable to changes in management practices and land use.

每年的农作物产量，土地使用变化和边际土地放牧等农业实践导致土壤碳（C）储量的损失。但是，土壤碳损失在农业系统中并不普遍，如果消除缺水等限制因素，土壤碳就会适度增加。为了描述这一点，我们使用了meta-对已发布数据的分析集中在半干旱地区，在该地区需要灌溉才能生产农作物。我们表明，与本地未灌溉的邻近草地或灌木丛相比，谷物，棉花，玉米和非木本园艺作物的土壤碳储量有所下降。相比之下，灌溉多年生木本多年生作物的种植通常会导致土壤C的积累。本研究的主要目标是确定在多年生木本多年生作物下的土壤中保留C的机制以及对C积累的任何限制。通过将土壤分为颗粒有机物（POM）和矿物相关有机物（MAOM），可以对土地利用发生变化时获得对土壤有机碳含量积累的机械理解。在这里，我们分析了C和自然丰度¹³用滴灌线灌溉的八个苹果园和八个葡萄园的土壤中，以及微喷的八个苹果园和八个樱桃园的土壤中POM和MAOM组分中的C浓度。还从邻近农业地点的八个原生草地地区取样以进行比较。与原生地相比，几十年的木本作物生产使土壤平均碳浓度翻了一番，在0-15厘米深度范围内从10.1±1.48 g C kg ^-1增至20.1±0.96 g C kg ^-1。大部分C与POM有关，浓度从7.9±1.19 g C kg ^-1增加到14.2±0.79 g C kg ^-1在0-15厘米的土壤中，增加了80％。这取决于作物，在樱桃园中最高，而在葡萄园土壤中最低。尽管含有较少的C，但MAOM浓度却增加了166％，从2.22±0.33 g C kg ^-1变为5.91±0.62 g C kg ^-1；作物之间没有差异，但是MAOM​​ C的浓度似乎被限制在〜12 g C kg ^-1的最大值。MAOM和POM具有显着不同的δ ¹³ C值：MAOM更富集，指示更大的微生物处理的，而POM具有较低的δ ¹³与主导地位植被一致C值。δ ¹³与本地站点相比，在农业站点的两个部分中的C值都被耗尽，这表明由于较高的C输入，在农业站点中积累了更多的较少处理的C。我们得出结论，该地区的土壤通过将C保留在POM和MAOM组分中，对灌溉的多年生木本作物产生了响应。作为MAOM的C的积累受到该地区土壤矿物学的限制，但不受占土壤C含量支配地位的POM的约束，并且可能易受管理做法和土地利用变化的影响。