Differences Of Soil Organic Carbon Between Orchard And Farmland In Hilly And Gully Region Of Southern Loess Plateau

Chang Wu County, located in the gully region of southern Loess Plateau, is a typical repesentitive zone of dry-land rain-fed agriculture in the transition zone of semi-humid to semi-arid climate. This region favors the growth of apple, and apple production has been growing rapidly in the recent 20years in this aera. Many crop fields have been converted into orchards, and apple has replaced crops as the main local agriculture production. Changes in land-use patterns are bound to affect the balance of soil carbon .The discussion about change of soil oraganic carbon (SOC) when farmland were transformed into orchards has important ecological and environmental significance on the swing of the pendulum of soil carbon pool of the Loess Plateau. Therefore, gully region of the Loess Plaetau in ChangWu County was chosen as the study object; samples were collected according to different landform sites (plateau land, slope land, and flood land) and land uses (farmland and orchard). Further, samples of typical soil profile were collected in the two different land uses (farmland and orchard).The changes and differences of SOC content, oxidation stability, combined state and storage of SOC were studied in this paper. The main conclusions are as follows:1. Soil organic carbon of Changwu ranged from 2.99 g·kg^-1 to10.59 g·kg^-1, and the average content was 7.29 g·kg^-1 .Its overall level reached medium class. The content of SOC varied from different topographic sites, and it ranked as: Plateau land (7.86 g·kg^-1)>Slope land (7.01 g·kg^-1)> flood land (6.38 g·kg^-1) in soil (0²0cm) and same trend was found in 20⁴0cm soil, namely Plaetau (6.23 g·kg^-1)>Slope (5.44 g·kg^-1)> flood land (5.06 g·kg^-1). There were significant differences existing among different topographic sites. The SOC content varied from farmland to orchard, and the average SOC was 7.54 g·kg^-1 for orchard and 7.48 g·kg^-1 for farmland. The SOC content of orchard is a little higher than that of farmland. The SOC content varied from planting history to history. In 0²0cm soil, SOC content of orchard with planting age more than 15 years demonstrated a descending trend. The average SOC content of orchard (<5a) was 7.62 g·kg^-1, 7.56g/kg for orchard with 5¹5a planting history and 7.45 g·kg^-1 for >15a planting history orchard. For 20⁴0cm soil, 5¹5a history planting history orchard was observed with the greatest SOC content (6.05 g·kg^-1), followed by <5a planting history orchard (5.95 g·kg^-1) and the lowest value was found in >15a planting history orchard. This result associated with general deep fertilization in orchard and more input of fertilizer in 5¹5a orchard during full bearing period.2. Within Changwu county, readily oxidized organic carbon (ROC) in farmland, orchards soil ranged from 1.83 g·kg^-1 to 7.06 g·kg^-1, and the overall average content was 4.86 g·kg^-1. ROC of different landforms ordered by plateau land (5.09 g·kg^-1)> slope land (4.52 g·kg^-1)> flood land (4.35 g·kg^-1), and significantly differed from each other. However, Oxidation stability coefficient Kos showed an opposite trend, where flood land (0.57)> slope land (0.55)> plateau land (0.54). Under different land use patterns, ROC content of cropland soil ranged from 1.83 g·kg^-1 to 7.61 g·kg^-1, with an average of 4.78 g·kg^-1; For orchard, ROC content varied from 1.86 g·kg^-1 to 7.06 g·kg^-1 and the average content was 4.89 g·kg^-1. The oxidation stability (Kos value) was farmland (0.56)>orchard (0.54). In terms of planting history, ROC of orchard ranked as"<5a orchard">">15a orchard">"5¹5a orchard". For oxidation stability of SOC, it was"5¹5a orchard (0.57)">">15a orchard (0.53)">"<5a orchard (0.51)"3. The SOC content change of HongJia plateau typical soil profile in ChangWu indicated no matter it is orchard or farmland, SOC demonstrated"S"shape change, namely SOC declined sharply in 0⁶0 soil, and increasely a little from 60¹20cm soil, then it declined sharply again from soil below 120cm. In general, change of total organic carbon (TOC) in the profile showed farmland> 5a Orchard> 5¹5a Orchard> 15a Orchard trend, ROC in the profile also showed the same trend. With the increasing depth of profile, percent of ROC to TOC declined while the ratio for non-readily oxidized organic carbon tended to increase followed the increasing soil depth. Relative content of both ROC and non-readily oxidized in 60¹20cm soil of 5¹5a orchard fluctuated considerably. SOC Oxidation stability (Kos) of soil profile increased as the soil depth goes deeper, and also increased with increasing planting history.4. Profile (0²00cm) distribution of organic carbon in storage and content shared the same trend, showing"S" type of trend. In 0 60cm soil, there were no significant differences between cropland, 5a fruit orchards and 5¹5a in organic carbon storage, which were 5.20 kg·m-2, 5.04 kg m-2 and 5.11 kg·m-2, respectively; but the orchard organic carbon over 15a history reserves at a low level (3.95 kg·m-2); In 60¹20 cm soil layer, SOC storage between farmland and orchard, between orchard of different planting history differed significantly. A trend was foound that 4.80 kg·m-2 (cropland) >4.49 kg·m-2 (orchard <5a)>3.98 kg·m-2 (5¹5a orchard) > 3.52 kg·m-2 (>15a orchard); in 120²00cm soil, there were no significant differences were found under the same condition. In the whole soil profile (0²00cm), the SOC storage ranked as 14.36 kg·m-2 (farmland) > 13.49 kg·m-2 (orchard <5a) > 13.01 kg·m-2 (5¹5a orchard)> 11.05 kg·m-2 (>15a orchard).5. The combined state of SOC indicated tightly combined organic carbon took the greatest proportion (68.9%) to TOC, followed by stably combined organic carbon (19.4%). Loosely combined organic carbon obtained relative low content accounting for 11.9% of TOC. The three forms of combined organic carbon showed varied performances according to different parts of terrain. Loosely combined organic carbon decreased by the order: flood land>slope land> plateau land; while stably organic carbon showed a contrasting trend: plateau land> slope land > flood land. For tightly combined organic carbon, the trend was plateau land> flood land >slope land. Comparison between farmland and orchard illustrated orchard was greater than farmland in Loosely combined organic carbon and stably combined organic carbon, while no significant difference was found in tightly combined organic carbon. In a typical profile of plateau, the ratio of loosely to tightly combined organic carbon of farmland was greater than that of orchard. Comprisons between orchards of different planting history indicated young orchard (<5a) took relatively higer content of loosely combined organic carbon in the whole profile while that of 5¹5a orchard and >15a orchard was lower. Stably combined organic carbon content was greater in 0 60 cm soils and old orchard of planting history greater than 15a; 5¹5a orchard had the greatest content of tightly combined organica carbon in the whole soil profile.