Effect of different tillage methods on soil organic carbon

Transcription

1 Ref: C0529 Effect of different tillage methods on soil organic carbon Philipp Bues and Eberhard Hartung, Institute of Agricultural Engineering, 6 Max-Eyth-Street, Kiel, (Germany) Abstract Numerous of the previously published tillage studies with subject on soil organic carbon (SOC) sequestration did very often - compare no-till (NT) with conventional tillage (CT). Results of these studies are contradictory and in many cases probably biased by sampling procedure, sampling depth respectively. Furthermore, only few studies have talked the impacts of reduced or minimum tillage on SOC stock. The objective of the current study was to assess how NT, CT and additional minimum tillage treatment with working depth of 15 cm (MT15) and 25 cm (MT25) affect SOC stock, bulk density and coarse pore space in the whole soil profile (0-100 cm). Measurements were made on a homogeneous loess-derived silt loam classified as Chernozem after 16 years operation of a still ongoing on-farm experiment in central Germany (Bernburg). SOC concentration results were converted to stock bases and equivalent soil mass approach. Distinctly differences among the tillage treatments were detected in all considered soil layers. Up to a depth of 24 cm, the NT management showed the highest SOC stock. In consideration of the whole soil profile, CT led to highest total SOC results while MT25 showed the lowest total SOC. The outcomes of the current study suggest that plough-less tillage does not store more SOC over the whole soil profile. At least under Bernburg soil and climate conditions a more or less only turning soil cultivation method with hardly any soil mixing like ploughing did result in a higher SOC stock than applying a cultivator with an intensive soil loosening by cultivation tines with a working depth of 25 cm. Hence, the relevance of the site specific actual working depth of minimum tillage strategies needs a stronger focus with respect to SOC. Keywords: tillage, soil organic carbon accumulation, long-term tillage experiment 1 Introduction Unquestionably soils are playing a major role in the global carbon cycle. For this reason agricultural activities which are causing SOC accumulation in soil are seen to have worldwide the most potential to mitigate the greenhouse gas concentration in the atmosphere (Smith et al., 2008). One of the most promising of these agricultural activities might be conservation tillage instead of conventional tillage (Pacala and Socolow, 2004). It is well documented that reduced tillage methods (plough-less tillage) compared to conventional tillage (CT) are able to conserve soil water, to reduce soil erosion, to save fuel, to reduce labor and under specific conditions to stabilize yields (Six et al., 2002). But does plough-less tillage really lead to higher SOC stocks? A wide range of prior studies on this subject did report high sequestration rates of SOC when adopting no-tillage (NT) (West and Post, 2002; Puget and Lal, 2005; Freibauer et al., 2004). More recently these findings are discussed more and more due to probably biased sampling procedure. Baker et al. (2007) assumed that the proclaimed high SOC sequestration potential of NT is just the product of a shallow sampling. Indeed most of the previously published studies did only sample to a maximum depth of 30 cm. Processes like root growth below the working horizon cannot be recorded that way. Furthermore, only Proceedings International Conference of Agricultural Engineering, Zurich, /7

2 few studies had examined the impacts of reduced or minimum tillage on SOC stock, instead of only focusing on SOC concentraitions. In respect of the marginally spread of NT and on the other hand the increasing relevance of reduced tillage systems in Europe is the SOC sequestration potential of these plough-less tillage treatments of high importance. The aim of the current study was at first to select a long-term tillage experiment with different intensities respectively working depths of reduced tillage treatments compared with CT and secondly to detect their effect on carbon storage in soil. 2 Materials and methods 2.1 Study site description A large scale trial was applied in 1996 with four different tillage treatments: i) plowing up to depth of 25 cm (CT); ii) minimum tillage with chisel cultivator up to a depth of 25 cm (MT25); iii) minimum tillage with disk harrow up to depth of 15 cm (MT15) and iv) no-till respectively direct drilling (NT). The experimental site was located at Saxony-Anhalt s State Institute for Agriculture, Forestry and Horticulture research station in Bernburg-Strenzfeld (51 49'N, 11 42'E), Germany. Soil at the site was a loess-derived silt loam classified as Chernozem on carbon-rich fluvial sand subsoil. The study area in the south of the Magdeburger Börde has a continental climate with a mean annual precipitation of 488 mm and mean annual temperature of 9.3 C. Since the beginning of the experiment in 1996, the site was rotational planted in sugar beet, winter wheat, winter barley, grain peas and winter rape. 2.2 Soil sampling and laboratory analysis In order to ensure that soil samples were taken at representative places in the study area, initially a soil map was created by using EM38 measurement (apparent electrical conductivity) combined with a GPS data logging. Based on this soil map sampling points were consistently distributed over the four tillage treatments and two conductivity classes using the GIS mapping software ArcMap 10 (Esri). For determination of bulk density, pore volume and pore size distribution undisturbed soil samples were taken in March For each tillage treatment, conductivity class and out of 1-5, 10-14, 20-24, and cm sampling depth always fourteen 100 cm³ cores were taken. Parallel, disturbed soil samples for SOC determination were taken from 0-10, 10-20, 20-30, 30-40, and cm depth, each variant and conductivity class. In order to calculate the SOC stock of different tillage methods the SOC content was multiplied by the corresponding bulk density. Afterwards it was corrected with an equivalent soil mass approach described by Ellert and Bettany (1995). For determination of pore volume, the undisturbed soil samples were saturated and subsequently drained by a pressure plate with pressures of -60 hpa, -300 hpa and -1,5 MPa. All data analysis where performed with SPSS Statistics 20 (IBM). Proceedings International Conference of Agricultural Engineering, Zurich, /7

3 3 Results The performed EM38-measurement indicated, that the soil at the experimental site is very homogeneous. The majority of the measured EM8-values were in a range of 15 to 23 ms/m (Milisiemens/meter). Due to soil homogeneity, the two conductivity classes had to be arranged very closely (18,1-18,5 and ms/m). That is why conductivity classes had no significant influence on SOC contend in this experiment. Therefore, the following results will not be differentiated by conductivity classes. Tillage intensity had a distinct impact on soil bulk density (Figure 1). Up to a depth of 24 cm the NT treatment had generally the highest and the CT and MT25 treatment the lowest bulk density. In the 1-5 cm depth interval the differences on soil bulk density between NT and CT as well as MT25 were highly significant; the MT25 variant ranged in this depth between NT and CT. In cm depth CT, MT25 and NT showed nearly the same bulk density and only MT15 resulted in a significant higher bulk density. Figure 1: Influence of tillage intensity on bulk density (g/cm³) in different soil depths (CT: plowing up to depth of 25 cm; MT25: minimum tillage with chisel cultivator up to a depth of 25 cm; MT15: minimum tillage with disk harrow up to depth of 15 cm; NT: no-till respectively direct drilling). With increasing soil depth, the total pore space decreased in all tillage treatments (Figures 2). In the cm interval the total pore space were lowest for all tillage systems. Between CT and MT25, only small differences occurred in 1-5, and cm depth, whereas between MT15 and NT only small differences in 1-5, and cm have been observed. MT25 led at all soil depth to higher total pore space than MT15 and NT, probably caused by the differences in mechanical impact and intensity as well as soil loosening and the associated soil aeration due to the used cultivation tools. Proceedings International Conference of Agricultural Engineering, Zurich, /7

4 Figure 2: Influence of tillage intensity on coarse pore space (%) in different soil depths (CT: plowing up to depth of 25 cm; MT25: minimum tillage with chisel cultivator up to a depth of 25 cm; MT15: minimum tillage with disk harrow up to depth of 15 cm; NT: no-till respectively direct drilling). In the upper 10 cm a clear gradation between the different tillage treatments with respect to SOC concentration (%) was detected (NT > MT15 > MT25 > CT; Figure 3). With increasing depth the SOC concentration decreased in all reduced tillage systems. Only the SOC concentration in the CT treatment has been steady till 24 cm sampling depth. From cm sampling depth the SOC concentration was about the same level in all treatments. Below a depth of 20 cm the CT treatment always showed the highest SOC concentration. Noticeable was the significantly lower SOC concentration of the MT25 treatment in the cm horizon, probably due to the strong mixing effect of the cultivation tools Proceedings International Conference of Agricultural Engineering, Zurich, /7

5 Figure 3: Influence of tillage intensity on SOC contend (%) in different soil depths (CT: plowing up to depth of 25 cm; MT25: minimum tillage with chisel cultivator up to a depth of 25 cm; MT15: minimum tillage with disk harrow up to depth of 15 cm; NT: no-till respectively direct drilling). Table 1 shows the SOC concentration expressed on area basis (Mg ha-1). Up to a depth of 24 cm the NT management increased SOC stocks in comparison to the other treatments. Comparing the SOC stocks up to a total depth of 100 cm, revealed clear differences between the tillage intensities with the gradation: CT > MT15 > NT > MT25. Table 1. SOC stock (means ± SD; CT: plowing up to depth of 25 cm; MT25: minimum tillage with chisel cultivator up to a depth of 25 cm; MT15: minimum tillage with disk harrow up to depth of 15 cm; NT: no-till respectively direct drilling). depth CT MT25 MT15 NT 0-6 cm (Mg ha -1 ) 1,13 ± 0,03 a 1,26 ± 0,08 b 1,34 ± 0,12 bc 1,46 ± 0,22 c 8-16 cm (Mg ha -1 ) 2,12 ± 0,09 a 2,09 ± 0,12 ab 2,18 ± 0,10 ab 2,21 ± 0,24 b cm (Mg ha -1 ) 1,62 ± 0,11 a 1,42 ± 0,15 b 1,66 ± 0,08 c 1,74 ± 0,09 d cm (Mg ha -1 ) 1,26 ± 0,20 a 0,95 ± 0,13 b 1,10 ± 0,09 c 1,24 ± 0,24 c cm (Mg ha -1 ) 5,00 ± 0,73 a 4,13 ± 0,19 b 4,57 ± 0,58 ab 3,73 ± 1,05 b cm (Mg ha -1 ) 3,41 ± 0,64 a 2,75 ± 1,04 a 2,62 ± 0,52 a 2,50 ± 0,76 a cm (Mg ha -1 ) 14,54 ± 1,8 12,60 ± 1,71 13,47 ± 1,17 12,88 ± 2,6 Taking in account the averaged crop rotation yield over the years 2008 to 2012, CT realized 102 %, MT15 98 % and NT 100 % level of crop yield (Table 2). Especially in the year 2010 with winter rape cultivation, the NT treatment showed positive effects on yield production. Proceedings International Conference of Agricultural Engineering, Zurich, /7

6 Table 2. Crop yields relative to reference base (mean of all tillage variants; CT: plowing up to depth of 25 cm; MT25: minimum tillage with chisel cultivator up to a depth of 25 cm; MT15: minimum tillage with disk harrow up to depth of 15 cm; NT: no-till respectively direct drilling). year crop rotation mean (dt ha -1 ) CT MT15 NT 2008 winter wheat winter wheat winter rape winter wheat winter wheat Discussions The bulk density results up to depth of 24 cm clarified, that increasing tillage intensity led to decreasing bulk densities or in other words compared to ploughing NT showed significantly higher bulk densities in the first 24 cm. These findings are consistent with Bescansa et al. (2006) and Gál et al. (2007) and due to the lack of mechanical soil loosing not surprisingly. In contrast to that, in the depth interval of 24 to 28 cm the CT and MT15 treatment showed the highest bulk densities, which is probably not due to tillage method but to an earlier plow sole made by the furrow wheel while ploughing. Although reduced tillage systems led to significant higher SOC stocks in the top soil, the total SOC stock over the entire soil profile did not increase with absence of plough. In fact the CT variant showed the highest total SOC stock which is in contrast to many prior findings (e.g. West and Post, 2002). One attempt to explain these results may be the increase in earthworm number and the resulting higher decomposition of SOC after implementation of reduced tillage management (VandenBygaart et al., 1999). Furthermore it is documented in literature that earthworms can destabilize macro aggregates which give access to unprotected SOC inside of the aggregates. The enlargement of soil surface and enhanced oxygen supply to microbial decomposers lead to a loss of previously occluded SOC (Denef et al., 2001). A good indicator for the input of organic substrates and thus for SOC sequestration potential is the site-specific yield, because higher crop productivity often causes an increasing root and crop residue mass. However, in this experiment the level of crop yield over the last 5 years has been higher in the NT treatment compared to the MT15 treatment, but the SOC stock was higher in the last-named treatment. A possible explanation why the MT25 variant has such low SOC values could be justified by the great volume of coarse pore space, probably caused by the mechanical impact, intensity and soil loosening of the used cultivation tools. This may let to an increasing soil aeration associated with increasing mineralization rates of SOC. 5 Conclusions The expectation that plough-less tillage generally results in a higher SOC stock in soil, could not be confirmed by the current results of the current study. Even more the current findings indicate that at least under Bernburg soil and climate conditions a more or less only turning soil cultivation method with hardly any soil mixing like ploughing did result in a higher SOC stock than applying a cultivator with an intensive soil loosening and the associated aeration by cultivation tines with a working depth of 25 cm. Hence, the relevance of the site specific actual working depth of minimum tillage strategies needs a stronger focus with respect to SOC. 6 Acknowledgements We would especially like to acknowledge and thank Dr. Joachim Bischoff (Landesanstalt für Landwirtschaft, Forsten und Gartenbau Sachsen-Anhalt) for giving access to the study site and his always helpful support. Proceedings International Conference of Agricultural Engineering, Zurich, /7

7 7 References Baker, J. M., Ochsner, T. E., Venterea, R. T., & Griffis, T. J. (2007). Tillage and soil carbon sequestration - What do we really know?. Agriculture, Ecosystems & Environment 118 (1-4), 1 5. Bescansa, P., Imaz, M. J., Virto, I., Enrique, A., & Hoogmoed, W. B. (2006): Soil water retention as affected by tillage and residue management in semiarid Spain. Soil and Tillage Research 87 (1), Denef, K., Six, J., Paustian, K., & Merckx, R. (2001): Importance of macroaggregate dynamics in controlling soil carbon stabilization: Short-term effects of physical disturbance induced by dry wet cycles. Soil Biology and Biochemistry 33 (15), Ellert, B. H., Bettany, J. R. (1995): Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Canadian Journal of Soil Science, Freibauer, A., Rounsevell, M. D. A., Smith, P., & Verhagen, J. (2004). Carbon sequestration in the agricultural soils of Europe. Geoderma 122 (1), Gál, A., Vyn, T. J., Michéli, E., Kladivko, E. J., & McFee, W. W. (2007). Soil carbon and nitrogen accumulation with long-term no-till versus moldboard plowing overestimated with tilled-zone sampling depths. Soil and Tillage Research 96 (1-2), Pacala, S., & Socolow, R. (2004). Stabilization Wedges: Solving the climate problem for the next 50 years with current technologies. Science 305 (5686), Puget, P., & Lal, R. (2005). Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use. Soil and Tillage Research 80 (1-2), Six, J., Feller, C., Denef, K., Ogle, S. M., de Moraes Sa, J. C., & Albrecht, A. (2002). Soil organic matter, biota and aggregation in temperate and tropical soils - Effects of no-tillage. Agronomie 22 (7-8), Smith, P., Martino, D., Cai, Z., Gwary, D., Janzen, H., & Kumar, P. (2008). Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society B: Biological Sciences 363 (1492), VandenBygaart, A. J., Protz, R., Tomlin, A. D., & Miller, J. J. (1999). Tillage system effects on near-surface soil morphology: Observations from the landscape to micro-scale in silt loam soils of southwestern Ontario. Soil and Tillage Research 51, West, T. O., & Post, W. M. (2002). Soil organic carbon sequestration rates by tillage and crop rotation: A global data analysis. Soil Science Society of America Journal 66, Proceedings International Conference of Agricultural Engineering, Zurich, /7