Nutrients fixation by algae and limiting factors of algal growth in flooded rice fields under semi-arid Mediterranean conditions: case study in Thessaloniki plain in Greece

Abstract

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

Analysis of Temporal Variation of Soil Salinity during the Growing Season in a Flooded Rice Field of Thessaloniki Plain-Greece

“…The effects of algae uptake on the concentration of K + in the flooding water are evident in Figure 2c where K + decreased at the beginning until the middle of the flooding season. This period corresponds to the highest algae biomass production and growth rates estimated by the GLEAMS-PADDY model [37]. Later in the season, K + increased again due to:…”

“…Water balance components were computed and analyzed in a previous study by Aschonitis et al [37] using the GLEAMS-PADDY model [28], which was modified to include algae growth in rice fields. The model was applied for the same experimental field at the same growing season, in order to assess the uptake of major nutrients by algae in the floodwater, the inflow by irrigation and the losses by runoff of N, P, Ca, Mg, K, Na.…”

“…Measurements of irrigation water inflow, surface runoff and ponding depth were used for the calibration of the model. The water balance components of irrigation inflow, precipitation, surface runoff (above the bunds of 10 cm height), drainage below the root zone, and actual crop evapotranspiration, were estimated by the model at 2260, 158, 520, 432 and 1350 mm, respectively [37]. The daily variation of ponding depth, irrigation water amount and precipitation are given in Figure 1a, while the daily variation of surface runoff, drainage and actual crop evapotranspiration are given in Figure 1b.…”

“…(Figure 2c). The application of the modified GLEAMS-PADDY model by Aschonits et al [37] showed that during the experimental period, the irrigation water provided 31.16 kg· ha −1 of K + while algae uptake from the flooding water was estimated at 39.58 kg· ha −1 . These results indicate that algae consumed more K + than the estimated inflow.…”

“…The development and application of models which describe salinity components (anions-cations dynamics) in rice fields, remain a great challenge even though many have been developed to describe the water, nutrients (nitrogen, phosphorus) and pesticides transport under these conditions [12,[25][26][27][28][29][30][31][32][33][34][35][36][37]. Methods based on water budget approaches in combination with the monitoring of salinity are common tools to analyze soil salinity components in rice fields [38][39][40][41].…”