Changes in soil properties and their effects on maize productivity following sesbania and pigeon pea improved fallow system in Eastern Zambia

Abstract

Soil degradation is a major constraint in most sub-Saharan African countries. short fallow rotations of 1-3 years have a potential to increase maize yield without additional inorganic nitrogen fertilizers. The mechanisms responsible for improved maize yield are partially understood. therefore the objective of the study were (1) to quantify some changes in soil properties that may be responsible for improvement in crop productivity under fallow cultivation systems compared with continuously cropped maize system, and (2) to quantify the nitrogen mineralization patterns when mixing litter(dry leaves or senesced leaves) and fresh leaves of fallow species. The experiment was laid out as a randomised complete bloke design with three replications on a sandyloam (typic kandiustalf) in eastern Zambia. the treatments compared were two-year-old planted improved fallow of sesbania sesban (L.) Merr. (S.sesban) and cajanus cajan (L.) Millsp (pigeon pea); respectively, natural vegetation fallow (Nf), continous fertized (M+F) and unfertilized (M-f) (zea mays L.) monoculture. The following parameters were measured: growth (performances) of trees, N mineralization patterns, soil mineralizable inorganic nitrogen, dry matter (DM) accumulation, maize yields, soil bulk density, soil porosity, soil organic carbon, soil penetration resistance, water aggregate stability and distribution, infiltration rate and soil water current. The highest survival seasons were recorded in S. sesban (92%) while pigeonpea had only (31%). at the end of the 10-week incubation period, the N mineralization of S.sesban (fres leaves and litter), reached 59.4mg N kg ־¹ soil for pigeonpea (litter). Natural fallow had a cumulative net immobilization of 0.8 mg N kg־¹ soil. M+f had the highest pre-season soil inorganic nitrate-N and total inorganic N in 0-20 cm soil depths but was not significantly different with pigeonpea and S. sesban land use systems (LUSs). A polynomial regression model between maize grain yield and pre-season soil inorganic nitrate-N for 0-20 cm, 0-40 cm and 0- 60cm soil layers showed that the amount of pre-season inorganic nitrate-N in the soil layer accounted for 70%, 67% and 69% respectively, of the maize yield. As was the case with pre-season soil nitrate-N, total inorganic N in 0- 20 cm and 0-60 cm soil depths were significantly correlated with grain yield (R² =0.70, 0.66 and 0.70, respectively).

The maximum N accumulation in maize above ground biomass at 24 WAP averaged 156.9kg N ha־¹ and 77.0 kg N ha־¹ M +f and the S.sesban, respectively, with grain yields of 5.51 and 3.02t ha־¹, correspondingly. The highest total biomass was recorded in the M+f (9.52 t ha־¹), followed by S. sesban (6.02), but was not significantly different at p≤0.05.

Penetrometer resistance measured at 4 WAP at 5 cm soil depth ranged from 0.65 MPa to 1.15 MPa for pigeonpea amd M-f land use systems, respectively. The highest wet mean weight diameter (WHWD) was recorded in the pigeonpea LUS at both fallow clearing and crop harvest. The percentage of water stable aggregates (>2.00mm) at fallow clearing was highest (83%) in S.sesban and lowest (61%) in M-f and land use systems. On the other hand at crop harvest the percentage aggregate (>2.00mm) was highest (77%) in pigeonpea and lowest (44%) in M-f land use systems. At fallow clearing, the equilibrium infiltration rate was highest (31.8 cm hr־¹) in natural vegetation and lowest (12.6cm hr־¹0 in m-f land use systems. On the other hand at crop harvest the equilibrium infiltration rate was highest (23.4 cm hr־¹) in S. sesban and lowest (10.2 cm hr־¹) In M-f. Similarly, cumulative water intake after 3 hours at fallow clearing was in order of: S.sesban> Nf=pigeonpea=M+f=M-f. soil water storage in 0-70 cm soil layer at 8 WAP was highest (236mm) in S.sesban and lowest (209mm) in M+f. the improved soil condition and nitrogen combination of S.sesban and pigeonpea fallows to subsequent crop was evidenced by increased maize yields after these fallows as compared to no tree treatments. Mixing of litter (low quality) with fresh leaves (high quality) from the same tree species at fallow clearing had an effect on maize N uptake. Therefore, there is no need to carefully manipulate the quantities of materials (fresh leaves and litter) at fallow clearing so as the maximum N utilization by maize plants in improved planted fallow systems.