Phosphorus (P) is an essential constituent for all living organisms, scarce with finite reserves. P is deficient in Sub Saharan Africa threatening primary production. That in part is due to the occlusion of Pin Fe and Al oxides in the weathered soils. This problem is aggravated by the high cost of P fertilizers, lack of both appropriate technologies for its application and government support to facilitate and strengthen its use in small-scale farming. Evaluation of soil P availability in the mixed-species plantations established on natural savannas in the Congolese coastal plains highlights that high C accretion involves low P availability in the mixed-species (50% acacia and 50% euca-lyptus) stands relative to others. P availability in 0-0.05 m in the mixed-species stands (6.94 +/- 0.45 mg kg(-1)) was lower than in acacia (8.07 +/- 0.63 mg kg(-1)) and eucalyptus (8.46 +/- 0.79 mg kg(-1)). C stock in the 0.25 m was higher in the mixed-species (17.8 +/- 0.7 t.ha(-1)) relative to acacia (16.7 t.ha(-1) +/- 0.4) and eucalyptus (15.9 t.ha(-1) +/- 0.4). However, afforesting the inherently nutrient-poor and sandy soils beneath savannas evidenced an improvement in soil P availability to plant, along with soil N status and C sequestration in both soil and biomass, with a potential impact on mitigating climate change. (C) 2019 Elsevier B.V. All rights reserved.
Mixed Eu- calyptus urophylla × grandis hybrid (18–52) and Acacia mangium stands were established in May 2004 (starter fertilization of 43 kg ha − 1 of N as ammoniumnitrate). Pure acacia (100A), mixed-species with 50% aca- cia and 50% eucalyptus trees (50A50E, the two species being alternately planted in the row and between adjacent rows) and pure eucalyptus (100E) stands were compared within a randomised block design with five replicates (planting density of 800 trees ha−1). Each stand (1250 m2) consisted of an inner plot comprising 36 trees (6 × 6) and two buffer rows (Fig. 1 adapted from Koutika and Mareschal, 2017). The first rotation ended after 7 years which is a full length rotation for euca- lyptus plantations established in the area. Trees were harvested in Jan- uary 2012. Using the same design, the second rotation was planted in March 2012, with a closely related Eucalyptus urophylla × grandis hybrid (18–147) and Acacia mangium but without any N fertilizer added, while potassium (K) was supplied three months after planting i.e., 50 kg ha−1 as KCl.
|Climate change impacts||Effect of Nbs on CCI||Effect measures|
|Reduced soil quality||Positive||Soil fertility: various measures of phosphorus in the soil in different size fractions of organic matter and total available P in the soil“"Introduction of nitrogen-fixing trees as a way to improve soil fertility led to an increase in standing wood biomass, soil C and N accretion in the mixed-species stands. It also resulted in a decrease of soil P availabil- ity relative to pure eucalyptus and acacia stands at the end of the first 7- year rotation on the Arenosols of the Congolese coastal plains. Even then, it must be noticed that overall soil P availability in afforested stands was still higher than those of natural savannas at year 3 of the second rotation. This study is showing that in the low input systems; such as the studied experimental site, afforesting savannas increases P availability of inherently nutrient-poor but low P fixing soil, along with an increase in N stock and a potential C sequestration in both soil and biomass."”|
The studied site is located on a plateau close to Tchissoko village in Congo (4°S, 12°E, 100 Alt.), on a deep Ferralic Arenosol laying on a bed- rock composed of thick detritic layers of continental origin dated from plio-pleistocene at about 35 km of Pointe-Noire city.