NITROGEN MINERALIZATION IN A SIMULATED RHIZOSPHERE AS INFLUENCED BY LOW MOLECULAR WEIGHT ORGANIC SUBSTANCES

Abstract

Rhizodeposits consist of over 200 organic compounds, mainly low-molecular-weight organic substances (LMWOS) such as amino acids (AA), carbohydrates (CH) and carboxylic acids (CA), lipids and phenols. Those LMWOS influence nutrient turnover, particularly N turnover. However, the exact influence of these organic substances on nitrogen mineralization is yet unknown. Therefore, the stimulatory effects of low molecular weight organic substances on nitrogen mineralization in the rhizosphere of a silty loam soil from Bangladesh were studied. A two week incubation experiment was set up consisting of four different treatments (in three replicates) with addition of artificial root exudates, namely (1) low molecular weight organic acids (a mixture of malic, tartaric, succinic, citric and lactic acid at a C-ratio of 80:9:5:4:2), (2) carbohydrates (glucose), (3) a mixture of amino acids (a mixture of histidine, valine, alanine and glycine at a C-ratio of 43:35:9:14) along with a (4) control. In this experiment a novel method was used for the simulation of rhizospheric environment for the localized application of the exudates and sampling of an artificial rhizosphere. For this purpose small perforated plastic straws were inserted into the soil columns and the soils in these straws, representing the rhizosphere was sampled separately. After 7 days of pre incubation at 25°C inside the incubator under saturated condition, root exudates were artificially injected inside the perforated straws at a rate of 103, 93 and 10 µg C g-1 soil week-1 for low molecular weight organic acids, carbohydrates and amino acids, respectively. N mineralization was assessed by extracting and measuring NO3- and NH4+ concentrations at regular time intervals. All LMWOS stimulated N mineralization in soil, but to a different degree. The highest N mineralization rate was obtained from amino acid amendment (5.69 mg N kg-1 soil day-1) followed by organic acids and carbohydrate (4.30 mg N kg-1 soil day-1) and the lowest from the control (3.31 g N kg-1 soil day-1). Our findings show that locally high concentrations of (artificial) root exudates indeed stimulate N mineralization.