Light Fraction Carbon and Water-Stable Aggregates in Black Soils

Pedosphere 17(1): 97-100, 2007ISSN 1002-0160/CN 32-1315/PPEDOSPHERE回2007 Soil Science Society of ChinaPublished by Elsevier Limited and Science Presswww. elsevier .com/ocate/pedosphereLight Fraction Carbon and Water -StableAggregates in Black Soils*lSHI Yi, CHEN Xin and SHEN Shan-MinKey Laboratory of Terrestrial Ecological Process, Instilute of Applied Ecology, Chinese Academy of Sciences, Shenyang110016 (China). E-mail: shiyi@iae.ac.cn .(Received April 17, 2006; revised November 19, 2006)ABSTRACTThe distribution of light fraction carbon (LF-C) in the various size classes of aggregates and its relationship to water-stable aggregates as well as the infuence of cultivation on the organic components in virgin and cultivated black soils werestudied by wet sieving and density separation methods. The total organic carbon (TOC) and LF-C were signifcantlyhigher (P≤0.05) in the virgin soils than in the cultivated soils. The LF-C in aggregates of diferent size classes variedfrom 0.9 to 2.5 g kg-1 in the cultivated soils and from 2.5 to 7.1 g kg~ 1 in the virgin soils, whereas the ratio of LF-C toTOC varied from 1.9% to 7.3% and from 5.0% to 12.2%, respectively. After being incubated under constant temperatureand controlled humidity for three months, the contribution of LF-C to TOC sharply decreased to an amount (1.7%- 8.5%)close to the level in soils that had been cultivated for 20 to 25 years (1.3%- -8.8%). As a result, the larger water-stablemacro -aggregates (especially > 1 mm) decreased sharply, indicating that the LF-C pool in virgin soils declined quicklyafter cultivation, which reduced the water stability of soil aggregates.Key Wornds: black soil, light fraction carbon, water-stable aggregatesCitation: Shi, Y, Chen, X. and Shen, S. M. 2007. Light fraction carbon and water -stable aggregates in black soils.Pedosphere. 17(1): 97-100.Because soil organic matter plays an important role in the formation and stabilization of soil struc-ture and can release its contained nutrients through mineralization in forms available to plants, one of;he main aims of sustainable agriculture is to maintain soil organic matter while improving its quality(Haynes, 1999; Li et al, 2000). Oades and Waters (1991) introduced the concept of aggregate hierar-chy in soils where organic material controlled aggregate stability. In agro-ecosystems, the turnover oflight fraction carbon (LF-C) is connected with the formation of macro-aggregates, and cultivation andfurrowing practices infuence the quantity of macro aggregates in cultivated soil layers (Oyedele et al,1999; Post and Kwon, 2000; Ni et al, 2004; Williams et al, 2005).Black soil distributed in Northeast China is considered as one of the most fertile soils in China, witha high fertility and superior physical and chemical characteristics. The black soil region is located inthe middle of Heilongjiang and Jilin provinces of northeast China. The total area of the region is about6 Mha. Most of the region, about 90% of total area, is located in Heilongjiang Province (Wen andLiang, 2001; Meng et al, 2003; Wang et al, 2003; Han et al, 2005). Because organic matter contentis relatively high and stable in black soils, it is dificult to detect total soil organic carbon changes inthe short term under diferent management practices (Gregorich et. al, 1994). However, LF-C of soilorganic matter is highly decomposable and can indicate中国煤化工nt residue input aswell as the seasonal change of organic matter.YHCNMHGIn this study the distribution of LF-C in the various size class aggregates of virgin and cultivatedblack soils was studied and an incubation study was carried out to investigate the water-stable aggregates*1Project supported by the State Key Basic Research Development Program (No. G1999011804) and the KnowledgeInnovatjgPR军m of the Chinese Academy of Sciences (No. KZCXZ SW-416).)8Y. SHI et al.and the dynamics of LF-C, as well as to evaluate the infuence of cultivation on the organic componentsin black soils.MATERIALS AND METHODSSoil samples were collected after the fall harvest in 2000 from the surface layer (0-10 cm) of acultivated soil (cultivated by 100 years) in Hailun (47° 26' N, 126° 38' E) along with virgin and cultivatedsoils (cultivated 20 and 25 years) in both Longahen (47° 40' N, 126° 32' E) and Zhaoguang (47° 00'N, 126° 49' E). All test soils were from the black soils, which have a fairly thick black soil layer andare rich in humus, in Suihua District of Heilongjiang Province. When samples were taken, the litterlayer was removed and soil samples were mixed on a piece of plastic to remove the roots. Under fieldwater conditions, aggregates in seven sizes classes (> 5, 5- 3, 3-2, 2-1, 1-0.5, 0.5-0.25 and < 0.25 mmin diameter) were collected through wet sieving and then dried at 50 °C for use.An incubation test was conducted under constant temperature and controlled moisture. Thirty gramsof a certain size class of dry aggregates was placed onto plate and distilled water was added until thewater content reached 40%. The samples were weighed and incubated at a constant temperature roomof 30 °C for about three months. During this period, water was added every 3- 4 days to maintain thewater content. Three months later, one group (including seven sizes classes of aggregates and each classin triplication) was chosen to measure the retention rate of the aggregates while another was dried at50 °C for use. The retention rate of aggregates (R) was calculated by:R= A/B x 100%(1)where A is the dry weight of a certain size class aggregate after incubation, and B is its dry weightbefore incubation.The LF-C was separated by density separation mnethod using a mixture of tribromomethane andethanol with a density of 1.8 as the separating agent (Lu, 1999). A Shimadzu TC-5000A was usedto measure total organic C (TOC) and heavy fraction C (HF-C) with LF-C being calculated from thediference between TOC and HF-C.Multiple comparison of the means was carried with one way ANOVA and LSD test.RESULTS AND DISCUSSIONDistribution profile of soil TOC and LF-CLF-C consists of undecomposed and partly decomposed root and plant fragments and charcoal par-ticles (Cadisch et al, 1996). After cultivation, total biomass of the black soil was about 60% of thatof the virgin soil, while the ratio of the underground part to the total changed from 72.9% to 30.9%-10.8% (Zeng et al, 1980). In this study, Table I showed that LF-C in virgin soils ranged from 2.5- -7.1g kg-1, accounting for 5.0%- -12.2% of the TOC, while that in cultivated soils was 0.9- 2.5 g kg ~ , ac-counting for 1.9%- 7.3% of the TOC. Thus, mean of LF-C in the cultivated soils was 64% lower thanTABLE ITotal organic carbon (TOC) and light fraction carbon (LF-C) in test soilsLocationSoil typeLand use patternTOCLF-CLF-C/TOCgkg-1%ZhaoguangVirgin soilNatural grassland中国煤化工12.2Longzhen5.0Cultivated soilCorn and soybeansMHCNMHG1.941.62士0.06b1.86.5Hailun33.55士0.13a2.467.3=)Mean士standard deviation; b)Means followed by the same letter in a column are not significantly diferent at P = 0.05by one way ANOVA and LSD test.LIGHT FRACTION C AND WATER-STABLE AGGREGATES9Sthat in the virgin soils, which was close to the published results (63.3%) in Six et al. (1998), and wascompatible with the change of total biomass after cultivation. Thus, it could be concluded that returnedplant residues in virgin soils were higher than those in cultivated soils, and that the reduction of organicmatter in cultivated soils was mainly due to the decrease in returned plant residues.Water-stable aggregate and relationships to TOC and LF-CTable II showed that LF-C (as a percentage of TOC) in different size class aggregates of virginsoils was higher than that in cultivated soils. This was because cultivation changed soil conditions andaccelerated decomposition of plant residues (Zhang et al, 1997), resulting in a loss of carbon that hadaccumulated in the macro-aggregates (Six et al, 2000).Jastrow et al. (1996) reported that the larger the aggregate, the more organic matter associatedwith it. In addition, Elliott (1986) indicated that in temperate grassland soils, more organic matterwas associated with macro-aggregates (> 0.25 mm in diameter) than that with micro-aggregates andthe organic matter associated with macro- aggregates was less stable than that with micro-aggregates.Puget et al. (1995) indicated that in macro-aggregates, the organic matter was younger and less stable.In this study, Table II showed that the more contribution of LF-C to TOC was in macro-aggregates,except 3-2 mm size class aggregates, compared with < 0.25 mm micro-aggregates.TABLE IILight fraction carbon (LF-C) for different size class aggregates as a percent of total organic carbon (TOC)LocationSoil typeAggregates size class>5mm5-3mm3-2mm2-1mm1-0.5mm0.5-0.25mm<0.25mm%-Zhaoguang Virgin soil11.36.74.26.09.08.84.9Cultivated soil3.11.31.8.57.95.13.2Virgin soil after incubation 3.15.86.9LongzhenVirgin soil13.211.76.10.818.035.711.1.92.4.76.8Virgin soil after incubation 5.74.7.42.5In virgin soils after incubation, no large change was found for TOC in water-stable macro- aggregateswhile contribution of LF-C to TOC sharply decreased to an amount (1.7%- -8.5%) close to the level in soilsthat had been cultivated for 20 to 25 years (1.3%- -8.8%). As the core fraction in water-stable macro-aggregates, LF-C decreased quickly after incubation. As a result, the water-stable macro-aggregates(especially > 1 mm) decreased sharply, except 5- 3 mm from Zhaoguang virgin soil (Fig. 1), which indi-80.0ab■Cultivated soil60.0840.020.00.0中国煤化工-0.02020.0 ,Y HCNMHGAggregate size classFig. 1 Retention rates of water-stable aggregates in black soils from Zhaoguang (a) and Longzhen (b) in Suihua Districtof Heilongjiang Province after the fall harvest in 2000.100Y. SHI et al.cated the positive effect of LF-C on the maintenance of water-stable macro-aggregates.According to the results of many other studies, the main reason for the consumption of organiccementing materials in aggregates is the reduction of returned plant residue and soil disturbance dueto furrowing. Furrowing makes soil organic matter decompose quickly, resulting in a decrease of water-stable aggregates. 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