Improving quantity and quality of soil organic matter (SOM) is pivotal for efficiency and sustainability gains in crop production. In the Brazilian Savannah (Cerrado), around 20-40% of total soil organic C under areas of soybean-based crop rotation and pasture has been regarded as of pyrogenic origin and resilient due to charred material (Roscoe et al., 2001; Jantalia et al., 2007). Here we investigated if SOM level is improved in a period of 4 aerobic rice seasons after application of wood-biochar (WB) in a clay and sandy soil in the Cerrado. The WB is a by-product of bioenergy production rich in pyrogenic C (70-80% of its weight). Pieces of charcoal made of Eucalyptus sp. timber via slow pyrolysis (450-550 ºC) which are smaller than 8 mm are not suitable for industrial or domestic uses and could be recycled as a soil amendment. High surface area due to its porosity (see Figure 1) and ion-charge characteristics (such as hydroxyl groups) can make WB very efficient sorbent for a range of organic chemicals which are of agronomic and environmental importance; but such characteristics may change over time due to a myriad of biogeochemical interactions that WB is likely to undergo in soil (Kookana et al., 2011).

SOIL ORGANIC MATTER IN A PERIOD OF 4 SEASONS AFTER APPLICATION OF WOOD-BIOCHAR INTO A CLAY AND SANDY SOIL IN A TROPICAL SAVANNAHMárcia T. de Melo Carvalho1, Beáta Emöke Madari2, Wesley G. de Oliveira Leal2, Diego Mendes de Souza2, Mellissa Ananias da Silva Soler2, Aline de Holanda Nunes Maia3, Fabiano André Petter4, João Carlos Medeiros2

1Embrapa Rice and Beans, PhD Candidate at Wageningen University and University of Tasmania, marcia.carvalho@embrapa.br, 2Embrapa Rice and Beans, Santo Antônio de Goiás, GO, beata.madari@embrapa.br, 3Embrapa Environment, Jaguariúna, SP, 4 Federal University of Mato Grosso, Sinop, MT

Introduction

In December 2008, a permanent field trial was established on a rain-fed sandy Dystric Plinthosol (17% clay, 76% sand, 7% silt) at Estrela do Sul Farm, in Nova Xavantina, Mato Grosso, Brazil. A similar experiment under center pivot irrigation was established in June 2009 on a clayey Rhodic Ferralsol (57% clay, 33% sand, 10% silt) at Embrapa Rice and Beans, Capivara Farm, in Santo Antônio de Goiás, Goiás, Brazil. Both experiments were set up as completely randomized block design. Plots of 40 m2 (4 m × 10 m) were arranged in four replications, with WB (0, 8, 16, 32 Mg ha-

1) and N-fertiliser (0, 30, 60, 90 kg ha-1), each applied in 4 levels. The WB was applied only once: in December 5, 2008 to the sandy soil; and in June 9, 2009 to the clay soil. The WB was milled to pass a 2-mm mesh sieve, spread manually over soil surface, and incorporated within 0.10-0.15 m soil depth using a harrow. After the establishment of the field trial, crop rotations were basically: rice (summer) - bean (winter) on the clay soil, and rice (summer) - fallow (winter) on the sandy soil. Rice was direct seeded. Crop residues and weeds were controlled using herbicide. Every plot received the same P and K rate together with the seed, and mineral N amounts were always fractioned at sowing and around 25 and 45 days after emergence. The SOM was determined by the Walkley‑Black method (Nelson and Sommers, 1996) using sulphuric acid to generate internal heat for the reaction. To identify continuous patterns of response of SOM to WB and N rate, response surface models were fitted to the data. Analyses were performed using the mixed procedure (Proc MIXED) of the statistical software SAS/STAT® (SAS Institute Inc., 2008), using location of plots as random effects and WB, N and WB×N as fixed effects.

Material and Methods

In the sandy soil, SOM increased with 0.07% per Mg ha-1 of WB applied at first season, followed by a quadratic response with minima at 16 Mg ha-1 at second season. There was no response to WB application at third and fourth season. To the contrary, in the clay soil, there was no response of SOM to WB application at first season. During second season, there was a quadratic response with minima at 16 Mg ha-1, followed by a linear increase with 0.26 and 0.23% per Mg ha-1 of WB applied at third and fourth season (Table 1). The organic matter added via crop residues might be the key component related to the relative increase in SOM in the clay soil, the opposite of what was observed in the sandy soil, where there was no crop rotation. In the sandy soil, SOM response to WB was positive immediately after establishment of the field trial, when the area of the experiment (a 15-years old pasture) was converted to agriculture. The WB used has also an oxidisable fraction (around 4% of the total C) which could also contribute to an immediate increase in SOM. In the irrigated clay soil, WB is possibly undergoing a weathering process which can contribute to an increase in C in oxidation states in the biochar surface, as suggested by Lin et al. (2012). On the other hand, the accumulation of SOM in the clay soil can also be a consequence of a physical protection within WB porosity, as indicated by Zimmerman et al. (2011). The effect of WB on SOM was even more significant during the fourth season (P≤0.05) than during the second and third season (P≤0.10). The original intrinsic porosity of the WB and the interaction of WB with the clay soil during the second season after WB application is presented in Fig. 1.

Results and Discussion

Figure 1 High resolution images of wood-biochar: the intrinsic porosity (400x) (left); and the interaction with soil particles at season 2 after application into a clay soil (5000x) (right).

Table 1 Parameter estimates (± standard error) for fitted surface response models representing the quantitative effect of wood-biochar (WB) and mineral nitrogen (N) rate on soil organic matter (SOM, g dm-3) within 0-0.2 m soil layer of a clayey Rhodic Ferralsol and a sandy District Plinthosol in a period of 4 growing seasons of aerobic rice after applying WB in the Brazilian Savannah.

Soil organic matter (SOM) increased with 0.07% per Mg ha-1 of WB only immediately after WB application in the sandy soil. To the contrary, in the clay soil, SOM increased with around 0.25% per Mg ha-1 of WB during third and fourth season after WB application. The effect of WB in increasing the SOM level in the clay soil is promising, raising questions about the mechanisms involved in this process and whether other soil physical properties, such as aggregation, is been affected by WB application.

Conclusions

JANTALIA, C.P.; RESCK, D.V.S.; ALVES, B.J.R.; ZOTARELLI, L.; URQUIAGA, S.; BODDEY, R.M. Tillage effect on C stocks of a clayey Oxisol under a soybean-based crop rotation in the Brazilian Cerrado region. Soil & Tillage Research, 95: 97-109, 2007.KOOKANA, R.S., SARMAH, A.K., VAN ZWIETEN, L., KRULL, E., SINGH, B. Biochar application to soil: agronomic and environmental benefits and unintended consequences. Advances in Agronomy,112: 103-143, 2011.LIN, Y., MUNROE, P., JOSEPH, S., KIMBER, S., VAN ZWIETEN, L. Nanoscale organo-mineral reactions of biochars in Ferralsol: an investigation using microscopy. Plant and Soil, 357: 369-380, 2012. NELSON, D.W., SOMMERS, L.E., 1996. Total carbon, organic carbon, and organic matter. In: Sparks, D.L., Page, A.L., Helmke, P.A., Loeppert, R.H., Soltanpour, P.N., Tabatabai, M.A., Johnston, C.T., Summer, M.E. (Eds.), Methods of soil analysis. Part 3 – chemical methods. Madison, Soil Science Society of America Inc., pp. 961 – 1010.PETTER, F.A. Biochar as soil amendment – agronomic and environmental aspects of its application in soils of the Brazilian savanna (Cerrado). Thesis (PhD in Agronomy: Plant Production) – School of Agronomy and Food Engineering, Federal University of Goiás, Goiânia, 2010. 129 p. ROSCOE, R.; BUURMAN, P.; VELTHORST, E.J.; VASCONCELLOS, C.A. 2001. Soil organic matter dynamics in density and particle size fractions as revealed by the C/C isotopic ratio in a Cerrado’s Oxisol. Geoderma, 104: 185-202.SAS INSTITUTE INC. SAS Version 9.2 © 2002-2008. Cary, NC, USA, 2008.ZIMMERMAN, A.R., GAO, B., AHN, M-Y. Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils. Soil Biology & Biochemistry, 43:1169-1179, 2011.

References

To the Brazilian Agricultural Research Corporation (Embrapa), for providing scholarship for the first author, funds and facilities for the establishment of field trials, data sampling and analysis. To CNPq for providing fellowship on research and development for the second author. To students Laissa Gonçalves, Leidimar Morais, Fabrício Ribeiro and Diogo Machado for helping with sampling. To State University of Mato Grosso (Unemat). We are thankful to Carlos Alberto Petter and family who supported the establishment of the field trial at Estrela do Sul Farm, Nova Xavantina, Mato Grosso, Brazil. We acknowledge the Institute of Physics at Federal University of Goiás (UFG) where high resolution images of wood-biochar were made.

Acknowledgements

Seasons Fitted models R2

-----------------------------------------------------------Sandy soil-------------------------------------------------1† 12.45 (0.10) +0.008 (0.005) WB* 0.212† 7.46 (0.31) -0.05 (0.03) WB* +0.04 (0.01) N*** +0.002 (0.001) WB2** -0.0003 (0.0001) N2*** 0.793 9.92 (0.70) -0.07 (0.03) N*** +0.0007 (0.0003) N2** 0.504 9.65 (1.53) +0.19 (0.08) N** -0.002 (0.001) N2 ** 0.38

-------------------------------------------------------------Clay soil--------------------------------------------------1 15.56 (0.32) +0.009 (0.005) N* 0.152 25.39 (1.16) -0.134 (0.099) WB ns +0.005 (0.003) WB2* 0.123 25.50 (0.67) +0.0668 (0.0339) WB* 0.164 29.87 (0.64) +0.0689 (0.0329) WB** 0.37Significance values for estimates: ***P ≤ 0.01, **P ≤ 0.05, *P ≤ 0.10, ns P > 0.10; R2: squared Pearson correlation coefficient between observed and predicted values. †Adapted from Petter et al. (2010).

50m 5m