Harvest residue effects on soil organic matter, nutrients and microbial biomass in eucalypt plantations in Kerala, India
Introduction
The area of forest plantations is increasing in both the tropical and subtropical parts of the world (FAO, 2010). New plantations are being established on different soil types, and under different climatic and management conditions. The productive capacity of any given site depends on its inherent fertility status and the management strategies adopted by the plantation growers (Tiarks et al., 1998). Poor site management practices have, in part, resulted in declining productivity of plantations over multiple rotations and gradual loss of soil fertility at many sites (Tiarks et al., 1998, Powers, 1999).
Eucalyptus is a common plantation forest species grown in tropical countries (including India) on soil types with a range of fertility status (Gonçalves et al., 1997, Sankaran et al., 1999, Laclau et al., 2010a, Laclau et al., 2010b). Eucalyptus is often the preferred species because it has a good capacity to grow productively under many conditions. However, lower productivity levels in 2nd and subsequent rotations have often been observed, and are typically attributed to poor site management practices such as extensive removal and/or burning of harvest residues (Gonçalves et al., 1997, Corbeels et al., 2005, Laclau et al., 2005) which results in the export of large amounts of nutrients from site in plant biomass at harvest of each rotation (Madeira and Pereira, 1991, Hopmans et al., 1993, Ludwig et al., 1997, Khanna, 1997, Sankaran et al., 2005, Corbeels et al., 2005). Such practices result in decline in soil fertility over multiple rotations and increase in rotation length to achieve economically harvestable stands. Retention of harvest residues in subsequent rotation crops potentially has advantages for sustaining and controlled release of nutrients during the plant growth period (Schaefer and Krieger, 1994, Johnson, 1995, Tiessen et al., 1994). Previous studies in Kerala have shown that there is potential for a large quantity of nutrients to be exported from Eucalyptus plantations at harvest, especially if key harvest residue fractions are substantially removed (Sankaran et al., 2005).
There has recently been increasing importance placed on maximizing the economic returns from short-rotation plantations. High input management is considered as an economic imperative (Herbert, 1996, Gonçalves et al., 2004, Smethurst et al., 2004), but it is not commonly practiced in plantation forestry across Eucalyptus growing areas in India because of high fertilizer cost and delayed return on investment (Gonçalves et al., 2004). Potential on-site options like harvest residue management are gaining increased prominence for soil fertility maintenance and supplementing the soil N (Mendham et al., 2003a, Mendham et al., 2003b, Tiarks and Ranger, 2008). Introduction of leguminous plant species is one of the potential options for increasing the system N (Gadgil, 1983, Mendham et al., 2004), while retention of harvest residues may be important for conserving site nutrient capital. The benefits of harvest residue retention and/or manipulation of organic matter have been well recognized in other plantation forestry systems (Jones et al., 1999, Saint-Andre et al., 2008, Laclau et al., 2010a, Laclau et al., 2010b, McKay, 2011, Voigtlaender et al., 2012, Mareschal et al., 2013).
Assessment of the impact of harvest residue manipulation on the rate of N-mineralization can be used to understand the potential effect on soil fertility and plantation productivity. Nitrogen deficiency is the most common nutritional limitation to many non-leguminous plantation species. Potentially available N, as determined through anaerobic incubation (Keeney and Bremmner, 1966) and sequential soil core incubation studies under field conditions (Raison et al., 1987) have been used for assessing the soil N availability. Likewise, long term laboratory incubation studies (Mendham et al., 2004, María et al., 2007, Sánchez et al., 2010, Mareschal et al., 2013) have provided insight into the N-mineralization pattern in different soils, harvest residue loads, and under variable climatic conditions. Models that account for changes in N mineralization with soil temperature and moisture also have been used to predict the pattern of N-mineralization under various soil–plant–climatic conditions (O’Connell et al., 2004, Mendham et al., 2009). The impact of retaining harvest residues on soil N mineralization, or on soil microbial biomass has not been previously reported for many tropical Eucalyptus plantations. The aim of this study was to understand the impact that harvest residue retention has on (i) the pools of soil carbon and nutrients, (ii) the dynamics of nitrogen and (iii) microbial biomass in tropical Eucalyptus plantations. We tested the hypothesis that residue retention would increase these soil attributes at four sites planted to E. grandis (2 sites) and E. tereticornis (2 sites) in Kerala, India. The study was part of a CIFOR network collaborative research initiative to explore the options for sustainable management of productive capacity of soils in multi-rotation plantations (Nambiar, 2008).
Section snippets
Experimental sites and soils
Details of the study sites are provided in Table 1, and have been described in previous publications (Sankaran et al., 2004, Mendham et al., 2004). In summary, the study sites comprised 2 lowland (E. tereticornis) sites, and 2 upland (E. grandis) sites on Ferralsol soils in Kerala, India. The plantations were established on sites where Eucalyptus had been grown for 2 rotations since 1977 (E. tereticornis sites), and 3 rotations since 1968 (Surianelli), and 1958 (Vattavada). The 4 sites
Soil C, N and P
Retention or removal of slash did not have any significant effect on soil C at any of the 4 sites (Fig. 1), and significantly influenced soil total N (Fig. 2) at only one of the 4 sites (significant for both 0–5 and 5–10 cm depth ranges at Punnala). Similarly, soil P (Fig. 3) was only influenced by harvest residue retention at one of the 4 sites (significant for the 0–5 cm depth at Surianelli). When tested across the sites, only P was significantly influenced by treatment.
Field experiments
Harvest residue
Total soil pools
We were not able to detect significant effects of residue retention on soil carbon, and only few changes on total soil N and P, probably partly because of the removal of only small amounts of nutrients compared to the total soil pools, and the variability in measurement of soil pools. For example, Sankaran et al. (2005) showed that there was 9–16 t/ha of total N at these sites (to 1 m depth), and 50–150 kg N/ha in the harvest residues, representing around 1% or less of the soil pools. Similarly
Acknowledgements
Funding for this research was provided by Australian Centre for International Agricultural Research (ACIAR). The authors thank the research fellows and technical personnel for the assistance in the field work and laboratory analysis of soil and plant samples. We also thank Sadanandan Nambiar, Philip Smethurst, Ayeska Hubner and Pat Mitchell for constructive comments on earlier versions of this manuscript. We also thank the anonymous reviewers for valuable comments that helped to improve the
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