MSc

A study entitled “Mineralization of soil nitrogen, carbon and kinetics of
enzymes under coconut based cropping system” was carried out during 2015-17
in the Department of Soil Science and Agricultural Chemistry, College of
Agriculture, Vellayani. The study was envisaged toassess the mineralization
potential of soil nitrogen and carbon and to evaluate the kinetic parameters of
agriculturally significant soil enzymes of coconut based cropping systems in mid
laterite soils of Agro Ecological Unit (AEU) 9. Soil samples were collected from
five coconut based cropping systems 1. Coconut + Fodder grass 2. Coconut +
Banana, 3.Coconut + Pepper, 4. Coconut + Tuber and 5. Coconut + Vegetable
under organic and conventional farming situations.
With regard to physical properties, the lowest bulk density of 1.30 Mg m-3
was noticed in Coconut + Vegetable under organic management. Coconut + Tuber
under organic system recorded the highest value for water holding capacity (19.86
%). The highest porosity was noticed under Coconut + Pepper (41.60 %) cropping
system, while the interaction effects of cropping and farming systems on porosity
was non significant. With regard to the electrochemical properties, the highest pH
of 5.33 was noticed in Coconut + Vegetable under organic management and the
lowest EC value of 0.26 dSm-1 was noticed in Coconut + Pepper and Coconut +
Tuber under conventional management system.
Analysis of available nutrient status revealed that the N and K status were
higher in Coconut + Vegetable cropping system under conventional management
viz., 326.14 kg ha-1 and 183.68 kg ha-1, while the highest value of available P
(91.87 kg ha-1) was noticed in Coconut + Fodder under organic management.
Among the secondary nutrients, calcium was noticed highest in Coconut +
Vegetable (310.59 mg kg-1) and magnesium in Coconut + Tuber (143.49 mg kg -1)
under organic management. Coconut + Banana under organic management system
recorded the highest available micronutrient status of Fe, Zn, Mn and Cu viz.,
45.19 mg kg-1, 1.64 mg kg-1, 23.29 mg kg-1 and 5.74 mg kg-1 respectively, while
Coconut + Pepper recorded the highest boron content (1.5 mg kg -1) in the
conventional management system.
Coconut + Banana cropping system under organic management situation
recorded the highest activities of major enzymes viz., dehydrogenase, acid
phosphatase and catalase i.e., 462.52 μg of TPF released g -1 of soil 24 h-1, 80.35
μg of p-nitrophenol released g -1 of soil 24 h-1 and 3.90 % of H2O2 hydrolysed g-1
of soil
h-1 respectively. In general, the enzyme activities were found to be the
highest in organic farming situation than the conventional farming situation. In the
case of β-glucosidase and amylase, Coconut + Pepper recorded the highest values
viz., 4.10 μgpnp D-glucosidase g-1 soil h-1 X 10-4 and 13.82 μΜ of maltose g-1 of
soil respectively.The highest values of bacterial count of 8.04 log cfu g -1 and
actinomycetes count of 4.30 log cfu g -1 were noticed in Coconut + Banana under
organic system of management.
Enzyme activity number (EAN) was calculated using the expression EAN
= 0.2{TPF+catalase (%)/10 + phenol (μg)/40 + amino-N (μg)/40 + amylase
(%)/20}.The highest value for EAN was noticed in Coconut + Banana under
organic system (94.01) followed by Coconut + Pepper (74.92). With respect to
mineralization of soil carbon, the highest values for organic carbon, water soluble
carbon and particulate organic carbon 1.63%, 72.74 mg kg -1and 0.75 %
respectively were noticed for Coconut + Tuber under organic management. The
highest microbial biomass carbon noticed in Coconut + Banana (15.11 mg of CO 2
g-1) might have contributed to the higher enzyme activity and thus higher enzyme
activity number (EAN). In the case of mineralization of soil nitrogen, Coconut +
Vegetable (organic) recorded the highest value of 281.80 mg kg -1 for
mineralizable nitrogen and 0.31% for total nitrogen thus contributing to14.95 %
of mineralizable nitrogen to total nitrogen. From the enzyme kinetics studies, V max
was found to be maximum in the 8th week for urease and acid phosphatase and 6th
week for dehydrogenase.
From the study, it is concluded that Coconut + Tuber and Coconut +
Vegetable cropping system under organic farming situation favours the
mineralization of carbon and nitrogen respectively. Coconut + Banana under
organic situation was found to be the best with respect to the soil biological
properties.