Evaluation of carbon allocation coefficients and net primary production for major crops in Khorasan-e Razavi Province

Increases in the concentration of CO2 in the atmosphere have prompted renewed interest in increasing the soil stocks of carbon in the agroecosystems to mitigate climate change and also improve quality of soil (14, 25).
Predicting the changes in carbon stocks of soil, therefore, depends on reliable estimates of net primary production (NPP) and the proportion of the NPP returned to the soil.
NPP defined as the increase in plant mass (biomass) plus losses (such as mortality, leaf abscission, herbivory, etc.), summed for both above- ground and below-ground tissues per unit area of land per unit of time. The annual NPP in agroecosystems and the distribution of carbon in for both above- ground and below-ground tissues of plant, is usually calculated from agricultural yield (3, 8, 17).
Our purposes were to estimate a set of coefficients for calculating below-ground NPP, above- ground NPP, total annual NPP, relative coefficients of carbon allocation, allocated carbon content and annual carbon inputs to soil for major agricultural crops in Khorasan-Razavi province.
Shoot and root biomasses for major agricultural crops in Khorasan-e Razavi province such as wheat, barley, corn, cotton, sugar beet, alfalfa and chickpea at plant maturity during two years 2015 and 2016 were measured. The crops in all studied fields were usually fertilized according to local recommendations. The numbers of sub-samples taken for shoot and root biomass measurements from 10 fields (which were subsequently averaged) were four. Roots by using cylinders roots (below ground biomass) were manually separated from the soil (10, 23, 29). After harvesting, shoots and root samples were separately dried to constant weight and expressed on a dry matter basis. Carbon content in each crop into four fractions including seed (CP), shoots (CS), roots (CR) and extra-roots (CE) were calculated in units of biomass carbon per unit area per unit of time (g C m-2 yr-1) (3, 9).
The results showed that relative coefficients of carbon allocation to different tissues, above-ground NPP, below-ground NPP and total annual NPP were significantly affected by different crop species. The highest contents of allocated carbon to above-ground and below ground tissues including seed (CP), shoot (CS), root (CR) and extra-root (CE) were observed for barley (4452.78 g C m-2 yr-1), alfalfa (8602.56 g C m-2 yr-1), alfalfa (2929.39 g C m-2 yr-1) and alfalfa (1904.11 g C m-2 yr-1), respectively. The maximum above-ground and below-ground NPP were calculated for barley and alfalfa with 12626.28 and 4833.5 g C m-2 yr-1, respectively.
Relative coefficients of carbon allocaion among different tissues of each crop is a useful approach for evaluation of soil carbon changes in agricultural systems. Indeed, carbon input and NPP are the most important variables for predicting the net rate of soil carbon changes