Abstract
For studying the effect of soil fertility management practices on N mineralization, urease activity and maize yield, replicated field trials were established in 2015 at Misamfu and Msekera agricultural research stations (ARS) representing two geo-climatic regions of Zambia. The soil at Msekera ARS is a sandy clay loam (SCL) from a Paleustult, while that at Misamfu is a loamy sand (LS) from a Kandiustult. The field trials had three categories of treatments namely legumes, traditional and conventional. The legumes group consisted of researcher-recommended legume-cereal intercrop systems of maize with Cajanus cajan, Crotalaria juncea and Tephrosia vogelii in combination with compound D (10% N, 20% P2O5, 10% K2O) and urea (46% N) at the recommended rate (200 kg ha-1) and half of the recommended rate (100 kg ha-1). Composted cattle manure and Fundikila, a special plant biomass management technique, were the inputs under the traditional category. The conventional category consisted of a treatment to which only chemical fertilizer was applied. Urease activity was determined in surface soil samples (0-20 cm) collected from the field trials after 3 years. For N mineralization, a laboratory incubation study was conducted over 13 weeks. For the laboratory incubation, an additional treatment to which no input was applied was included as control. Application of organic inputs significantly increased the potentially mineralizable N (No) by 127% to 256% on the LS and by 51% to 131% on the SCL in comparison to the control. Similarly, the cumulative N mineralized (Ncum) was twice or thrice higher where organic inputs had been applied in comparison to the control. The No followed the order traditional > legumes > conventional > control, while the mineralization rate constant (k) followed the order legumes > conventional > traditional > control on both soils. The rate of N mineralization was significantly higher on the LS than the SCL. Higher rates of chemical fertilizer resulted in high Ncum and higher maize yield. Maize yield was significantly and positively correlated to Ncum, but inversely correlated to the amount of applied N that was mineralized (%Nmin). Urease activity was stimulated by application of organic inputs and suppressed by higher rates of chemical fertilizers. The type of organic inputs; the rate of chemical fertilizers; and soil texture are factors influencing N mineralization and maize yield. Urease activity was largely influenced by the rate of chemical fertilizer, but not the type of organic inputs or soil texture.
Highlights
Nitrogen is probably the most important nutrient for crop production as it is normally taken-up in higher amounts than any other nutrient (Bhat, Saroa, Benbi, Choudary, & Padder, 2015), and is known to limit primary production in most terrestrial ecosystems (Sekaran, McCoy, Kumar, & Subramanian, 2019; Cartes, Alejandra, Damanet, & Mora, 2009)
The lower yields associated with the combined application of organic inputs with lower or zero rates of chemical fertilizer at both agricultural research stations (ARS) are indicative of the fact that organic inputs, due to their slow decomposition rates might not be adequate to supply the required crop nutrients in the short-term
In the first year, there was no organic matter returned to the soil from the leguminous shrubs, i.e., the pigeon pea and tephrosia at both ARSs (Table 2) implying that the maize crop in these treatments was solely supported by the nutrients from the chemical fertilizer
Summary
Nitrogen is probably the most important nutrient for crop production as it is normally taken-up in higher amounts than any other nutrient (Bhat, Saroa, Benbi, Choudary, & Padder, 2015), and is known to limit primary production in most terrestrial ecosystems (Sekaran, McCoy, Kumar, & Subramanian, 2019; Cartes, Alejandra, Damanet, & Mora, 2009). Large organic molecules are broken down by hydrolytic enzymes that transform the organic N to plant available forms (Karuku & Mochoge, 2018). There are many different hydrolytic enzymes in the soil that make nutrients available to plants. Depending on their location, these enzymes can be categorized as extracellular or intra-cellular (Srinivasa-Rao et al, 2017; Piotrowska-Dlugosz, 2014). Among many soil enzymes that catalyse different soil biological processes, urease, an extracellular enzyme, is important in the nitrogen cycle (Dilly, Blume & Munch, 2003; Piotrowska-Dlugosz, 2014) as it is involved in the hydrolytic conversion of organic N into plant available forms. An understanding of the influence of different soil fertility management practices on the activity of urease is vital in understanding the N mineralization process
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