Soil P availability under eucalypt and acacia on Ferralic Arenosols, republic of the Congo
Introduction
The productivity of fast-growing tree plantations declines in second and subsequent rotations owing to a reduction in soil fertility when nutrients exported at harvest are not supplied as fertilizers (Corbeels et al., 2005, Laclau et al., 2005, Nambiar and Kallio, 2008, Tiarks and Ranger, 2008). In the Congolese coastal plains, eucalypt plantations deplete soil fertility in the highly-weathered inherently nutrient poor sandy Ferralitic soils, due to the large nutrient exported in harvested biomass, and the subsequent leaching of nutrients after harvest at the end of each rotation (Laclau et al., 2005). These soils are typical of large areas of savannah in central Africa, especially in the Batéké Plateaux spanning over 6 million hectares in Gabon, the republic of the Congo and the DR Congo (Schwartz and Namri, 2002). The retention of harvest residues (Corbeels et al., 2005, Kumaraswamy et al., 2014), the plantation of trees fixing nitrogen from the atmosphere, so called N2 fixing species (NFS) such as fast-growing exotic acacia species or their introduction in eucalypt plantations may improve soil fertility by reducing N depletion and sustaining forest productivity (Khanna, 1998, Binkley et al., 2000, Forrester et al., 2006, Epron et al., 2013, Koutika et al., 2014). These practices have been implemented in the Congolese coastal plains in the republic of the Congo (Nzila et al., 2002, Bouillet et al., 2013) as well in the Batéké Plateaux in the DR Congo (Kasongo et al., 2009).
In addition to soil N supply, phosphorus (P) is an essential nutrient for forest productivity in most tropical soils due to its major role in biological nutrient cycling (Hinsinger, 2001, Yang et al., 2013). Without regarding P export and potential leaching at harvest, soil P availability to plants is greatly reduced by strong adsorption of P due to the large amounts of Al and Fe oxide surfaces present in weathered soils (Sanchez and Uehara, 1980, Hinsinger, 2001). Soil P availability is especially critical in mixed-species plantations containing NFS owing to the high P requirement for symbiotic fixation of atmospheric N2 (Binkley, 1992, Hinsinger, 2001, Inagaki et al., 2011). Thus, increasing N availability in mixed-species plantations including NSF trees may shift the system from N-limitation to P-limitation, with a potential negative feedback on N2 fixation after several rotations, even though the phosphatase activity of NFS may enhance P availability (Blaser et al., 2014).
Due to the low input of fertilizers in commercial forest plantations established on the Congolese coastal plains, soil fertility, tree nutrition and early growth mainly rely on the decomposition of organic residues (Laclau et al., 2005, Versini et al., 2013). During the juvenile stage of a eucalypt plantation up to canopy closure, the uptake of nutrients from the soil reserves supplied most of tree growth requirements (Laclau et al., 2003). At a site located on the coastal plains of the Congo, previous results of the first rotation (2004–2011) of a mixed plantation of acacia and eucalypt have shown that eucalypt growth benefits from the N2 fixed by acacia (Bouillet et al., 2013, Epron et al., 2013). However, a decrease in soil resin P in the topsoil of the mixed-species stands relative to the pure eucalypt stands was observed at the end of the first 7-year rotation (EndR1) (Koutika et al., 2014).
These results obtained at the end of the first rotation raise concerns about the relationship between soil fertility e.g., P availability and the sustainability of mixed-species plantations of acacia and eucalypt. Two questions have emerged: (1) would the decrease in resin P values observed in the mixed-species stands (half eucalypt and half acacia) at the end of the first 7-year rotation amplify at year 2 of the second rotation? (2) Will a low available soil P decrease P uptake by NFS trees during the first 2 years of the second rotation? In this study, soil P availability at two key stages of eucalypt–acacia plantation rotations (EndR1 and Y2R2) was compared. Soil P availability was characterized by quantifying the resin-extractable P, Pi-HCO3 and Po-HCO3 fractions from the Hedley soil P sequential extraction procedure that are considered readily available to plants (Tiessen et al., 1984, Tiessen and Moir, 2008). The amount of P in the tree biomass and in litterfall was also estimated at year 2 of the second rotation, a juvenile stage when the nutrients uptake from the soil reserves supplied most of tree growth requirements, especially in low-input systems.
Section snippets
Site description
The study site is located about 35 km outside Pointe-Noire city on the coastal plains close to Tchissoko village in the Republic of Congo (4° 44′ 41″S & 12° 01′ 51″ E, 100 Alt.). The climate of the area is subequatorial with high mean annual air humidity and air temperature (85% and 25 °C, respectively) and low seasonal variation (about 2% and 5 °C, respectively). Annual precipitation averages 1200 mm with a dry season extending from June to September. The soils in this area are deep Ferralic
Soil resin P
The values of soil resin P down to 15 cm in 50A50E and 100E were significantly higher at Y2R2 relative to EndR1, while those of 100A did not significantly change with stand age (Fig. 2). At Y2R2, soil resin P down to 15 cm in 100E was significantly higher than those in 50A50E, while the latter was also significantly higher than those in 100A (Fig. 2). There was also more resin P in the 5–10 and 10–15 cm layers than in the 0–5 cm layer in all stands and at both EndR1 and Y2R2. This leads to stocks
Discussion
Our previous studies of the first rotation (2004–2011) of a mixed-species plantation of acacia and eucalypt established on Ferralitic Arenosols of the Congolese coastal plains reveal that eucalypt growth benefits from the N2 fixed by acacia (Bouillet et al., 2013, Epron et al., 2013), even though soil resin available P decrease in the topsoil of the mixed-species stands relative to the pure eucalypt stands at the end of the first 7-year rotation (Koutika et al., 2014). Studies conducted on
Acknowledgement
The authors thank Sylvain Ngoyi, Alpiche Diamesso and Tiburce Matsoumbou (CRDPI, Congo) for their field and laboratory assistance, Joelle Gerard (UMR, EEF, Nancy France) for assistance in providing laboratory facilities for Hedley P fractionation, P extraction and analyses, Christian de Cayeux and Tim Crews (Land Institute, USA) for their editing in the earlier stage of the paper, Hugo Rainey (WCS) for checking English, Paul Withers (Bangor University, Wales) for its contribution and the two
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