How do climate and land-use affect soil carbon and nitrogen stocks and the chemical properties of floodplain soils in tropical drylands?

•Dry sub-humid floodplain soils accumulated 15 % more C than soils under semiarid.•Dry sub-humid climate favored the largest natural fertility of floodplain soils.•Overgrazing causes more negative impacts on floodplain soils than agriculture.•Overgrazing reduced natural C and N stocks in floodplain soils by 65 and 62 %. Floodplain soils represent important food production sites worldwide due to their high natural fertility and large carbon (C) storage capacity. In this study, 27 soil profiles, along an aridity gradient, were used to estimate how climate (semi-arid and dry sub-humid) and land-use (agriculture and overgrazing) affect soil C and nitrogen (N) storage and chemical properties of surface horizons in floodplain soils in tropical drylands, northeastern Brazil. The results indicated that soils under dry sub-humid climate (wetter) accumulated 15.1 and 17.0 % more C and N than soils under semi-arid climate (drier), respectively. The Ca2+, Mg2+, K+, Na+, H+ contents and P availability of the soils were favored in the wetter condition. In the land-use analysis, the results indicated that soils under overgrazing showed lower C and N stocks by 35.7 % and 36.7 % respectively, when compared to soils managed under agriculture. Soils under overgrazing presented, on average, 36.0, 44.4 and 26.0 % less Ca2+, K+ and CEC than the soils under agriculture, respectively. For P, both overgrazing and agriculture reduced its availability by 21.2 and 31.4 % respectively, when compared to the forest soil (reference). The significant effect of overgrazing was evident in the reduction of C and N stocks and natural fertility of the floodplain soils. The results of this study can be useful for the correct management and definition of strategies for sustainable uses of floodplain soils in drylands globally.