The aim of this study is to assess the fixation of the major nutrients C, N, P, Ca, Mg, K, Na by algal biomass produced in the rice fields of Thessaloniki plain in Greece under semi-arid Mediterranean conditions and to evaluate the limiting factors for their growth. Measurements were performed in experimental rice-field following the regional conventional practices (C-H treatment): (a) direct sowing, (b) continuous flooding with few intermissions, (c) use of nitrogen fertilizers at 176 kg N ha-1, and (d) application of herbicides (active ingredients benzofenap and clomazone). Herbicides were not applied in a small part of the field isolated by bunds (C-NH treatment) in order to assess possible limiting effects of herbicides on algae growth. Climatic data, measurements of rice crop characteristics and water quality of the ponded water in the C-H part of the experimental field were also obtained in order to assess the limiting effects of light, temperature and nutrients based on a modelling approach. Green algae were found to be dominant in the specific system. Considering the two treatments, the results showed that herbicides did not affect algae growth probably due to the short period of exposure followed by the continuous flooding. Nutrients fixation by algae for C-H followed the order C (52.1 %) > Ca (5.6 %) > K (3.5 %) > N (2.4 %) > Mg (0.3 %) ≈ Na (0.3 %) > P (0.24 %) with a final dry biomass production at 1,118 kg ha-1. Based on the measurements and model simulations the most limiting factors under the regional conventional practices of rice cultivation were the temperature at the initial and final stage of rice growing season, the light when the leaf area index of rice was >2 and phosphorus concentration in the ponded water. The mean algae growth rate during the flooding period was estimated at 8.2 kg ha-1 day-1, while the maximum rate was estimated at 15.9 kg ha-1 day-1 at the initial growth stages of rice before the beginning of intense light limitations from rice crop coverage
Summary
The objective of this study is to present a new method for field capacity estimations using the CPV function (changes in pore volume). The method requires water retention data of an aggregated soil before and after a compaction event. The CPV function estimates the threshold of equivalent pore diameter (D‐threshold) between structural and textural porosity using changes in the water retention curve. The D‐threshold corresponds to a water potential associated with field capacity according to the TFC concept (textural field capacity), which assumes that (i) the textural pores define the field capacity conditions and (ii) the D‐threshold and consequently the water potential at field capacity are constant parameters for each soil regardless of its compaction state. Published datasets for different aggregated soils subjected to different compaction processes and data from a lysimeter drainage experiment were used to evaluate the method. The analysis of the drainage experiment was supported by the BUDGETv.6.2 model, which was used to estimate the optimum value of the water content at field capacity (parameter of the model) using observed soil water contents and drainage rates during the drainage experiment (concept of inverse problem). The results for different soils subjected to compaction showed that the estimated water potentials by the CPV method follow the commonly accepted assumptions that govern field capacity conditions, justifying also the assumptions of the TFC concept. The field capacity conditions estimated by the model demonstrated an adequate correspondence to the results of the CPV method, indicating its ability to assess the field capacity conditions of the lysimeter soils.
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