The 4 per 1000 initiative.

Soil organic matter is at the nexus of global challenges: food security, climate change adaptation and mitigation, soil security. The 4 per 1000 initiative, launched at the Climate COP21 within the Lima-Paris Action Agenda proposes to increase soil organic carbon (SOC) stocks to simultaneously address all these challenges. It directly addresses three sustainable development goals: SDG2 ?no hunger?, SDG13 ?Climate action?, and SDG15 ?Life on land? and indirectly concerns several others. The initiative targets agricultural soils in priority, which are often the most degraded soils and because of the high expected benefits in terms of soil fertility and hence of productivity. A range of agricultural practices are available that allow to increase SOC stocks while ensuring a resilient, productive and environmentally friendly agriculture, so that a large-scale deployment can be aimed at. Here, we review and discuss the main limits and criticisms addressed to the 4 per 1000 initiative

Nutrient concentrations and NH4+‐N mineralization under different soil types and fallow forms in southern Cameroon

To evaluate the soil‐fertility sustainability of the fallow systems, nutrient concentrations and NH4+‐N mineralization were determined in different soil and fallow types in the humid forest zone of southern Cameroon. Two experiments were conducted, the first comprised planted leguminous tree Calliandra calothyrsus, planted leguminous Pueraria phaseoloides, and regrowth mainly composed of Chromolaena odorata on the Typic Kandiudult. The second experiment made up of a fallow dominated by C. odorata, a fallow with C. odorata removed, and a P. phaseoloides fallow on the Rhodic Kandiudult, Typic Kandiudult, and Typic Kandiudox. In the first experiment, available P, Ca2+, K+ concentrations and effective CEC under C. calothyrsus were, respectively, 40%, 22%, 45%, and 15% lower when compared to P. phaseoloides but no differences were found between soils under P. phaseoloides and C. odorata. Mineralization of NH4+‐N was higher under C. calothyrsus than under C. odorata‐ and P. phaseoloides‐fallow types, indicating the impoverishment of organic material under the former. In the second experiment, the beneficial effect of P. phaseoloides was found in the Rhodic Kandiudult in the 0–10 cm layer throughout its low NH4+ release from mineralization. In the Typic Kandiudult, no differences in NH4+‐N mineralization were found between C. odorata and P. phaseoloides fallows. In the Typic Kandiudox, there was no difference in NH4+ mineralization between the three fallow types. According to the nutrient concentrations and NH4+ mineralization, the fertility sustainability of the different fallow types may be ranked as follow: P. phaseoloides ≥ C. odorata > C. calothyrsus > fallow without C. odorata

Soil organic matter assessment in natural regrowth, Pueraria phaseoloides and Mucuna pruriens fallow

Published online: 9 May 2001Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in southern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM), soil exchangeable Ca2+, Mg2+ and K+ and available P concentrations, effective cation exchange capacity (ECEC) and, soil acidity in the 0–10 and 10–20 cm layers were evaluated under: natural regrowth mainly composed of Chromolaena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mineralisation during a 4-week incubation at 28°C in the laboratory. In addition, particulate organic matter (POM), the most active part of SOM, was fractionated by wet sieving into coarse (4000–2000 μm), medium (2000–250 μm) and fine (250–53 μm) particle size classes and analysed for C and N contents. Under Mucuna, Ca2+, K+ and P concentrations, ECEC and soil pH were higher and C mineralisation was lower than under natural regrowth and Pueraria in 0–10 cm depth. Soil under natural regrowth had a higher C mineralisation in 0–10 cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM, for any of the fractions between the three fallow types. However, both leguminous fallow species increased POM quality through a higher N content. Compared to natural regrowth, Pueraria increased N content in coarse POM by 36% in the 0–10 cm layer and by 19% (coarse POM) and 35% (medium POM) in the 10–20 cm layer. Mucuna increased N content in the 0–10 cm layer by 12% (coarse POM), and by 19% (fine POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SOM, the three fallow types ranked: Mucuna≥Pueraria>natural regrowth. However, in terms of POM quality the ranking was: Pueraria>Mucuna>natural regrowth

Microbial biomass P dynamics and sequential P fractionation in high and low P fixing Kenyan soils

We tested how the incorporation of farmyard manure (FYM) and inorganic phosphorus (P), KH2PO4 fertilizer changed soil microbial biomass phosphorus (biomass Po and inorganic and organic P fractions) in high and low P fixing Kenyan soils. Four soil treatments were compared during 32 weeks of laboratory incubation: (i) Nil; (ii) FYM (10 g kg−1); (iii) KH2PO4 (2.5 g kg−1) (P); and (iv) FYM (10 g kg−1) + KH2PO4 (2.5 g P kg−1). Biomass P was determined by fumigation-extraction followed by resin extraction, and P availability determined using a modified Hedley P fractionation method. The addition of FYM + KH2PO4 only increased biomass P in the high P fixing soil. From a maximum of about 22.5 μg P g−1 soil (week 1), biomass P declined to 4.8 (week 2), increased steadily to week 16 (15.2), and then decreased to 9.7 μg P g−1 soil (week 32). Biomass P in the low P fixing soil, with higher fertility, resin P and organic matter concentrations did not show any significant change. The P fractions that changed the most relative to the control, were the resin, Po–HCO3 and P–OH (week 1 and 16). This difference was enhanced in the high P fixing soil but resin P was the same in that soil at week 16, and the reduction in Po–OH in the FYM + KH2PO4 treatment (162.4 μg P g−1) compared to the control (242.2 μg P g−1) was more pronounced. This indicates an enhancement of microbial activity in the high P fixing soil