Microprofiling of nitrogen patches in paddy soil: Analysis of spatiotemporal nutrient heterogeneity at the microscale

Abstract

Flooded paddy soil ecosystems in the tropics support the cultivation of the majority of the world's leading crop, rice, and nitrogen (N) availability in the paddy-soil rooting zone limits rice production more than any other nutritional factor. Yet, little is known about the dynamic response of paddy soil to N-fertiliser application, in terms of horizontal and vertical patchiness in N distribution and transformation. Here, we present a microscale analysis of the profile of ammonium (NH 4 + ) and nitrate (NO 3 − ), nitrification, oxygen (O 2water and O 2soil ), and pH (pH water and pH soil ) in paddy soils, collected from two representative rice-production areas in subtropical China. NH 4 + and NO 3 − exhibited dramatic spatiotemporal profiles within N patches on the microscale. We show that pH soil became constant at 1.0-3.5 mm depth, and O 2soil became undetectable at 1.7-4.0 mm. Fertiliser application significantly increased pH, and decreased O 2 , within N patches. Path analysis showed that the factors governing nitrification scaled in the order: pH water > pH soil > NH 4 + > O 2water > NO 3 − > O 2soil . We discuss the soil properties that decide the degree of nutrient patchiness within them and argue that such knowledge is critical to intelligent appraisals of nutrient-use efficiencies in the field.It is now well established more generally that soil nutrients, including N, are distributed in a heterogeneous or patchy manner within ecosystems 1,2 due to a combination of natural and anthropogenic factors. In agricultural soils, N fertiliser application is the main anthropogenic driver that produces heterogeneity in soil N distribution, and fundamentally affects local N pools and N-cycling processes within soil 3 . Recent research on soil heterogeneity has almost exclusively focused on plant behavior. When roots encounter a nutrient-rich zone or patch, they often proliferate within it, including increases in elongation of individual roots 4 ; total root length 5 ; root production 6 ; and extent of lateral branching 7 . These plastic responses by the root system have been proposed as the major mechanism employed by the root system to allow plants to cope with the heterogeneous supply of nutrients in soil 1 . Variations in both root biomass and N uptake rate per unit root biomass are important in contributing to the variations in the abilities of species to capture N from ephemeral patches 8 . Field studies have shown that plant roots respond most strongly to N given in pulses and least strongly to a continuous nutrient supply 9 . Furthermore, there was a positive relationship between N uptake rate, relative growth rate, and root system biomass 8 . In addition to the plastic responses of plant roots, there have been reports that soil N heterogeneity influences seedling recruitment 10 , vegetation succession 11 , plant species coexistence and competition 12 , and invasion of non-native plants into natural ecosystems 13 .In flooded paddy soils, N is applied principally as ammonium (NH 4 + )-based or urea fertiliser...

Nitrogen Addition Decreases Dissimilatory Nitrate Reduction to Ammonium in Rice Paddies

“…Ϫ availability is generally low in rice paddies (4). However, localized oxic conditions in the rice rhizosphere, due to the leakage of O 2 from the roots, enables nitrification and, thus, a continuous supply of NO 3…”

“…Ϫ (16,17). Rice paddies are low in NO 3 Ϫ because the anaerobic conditions in the soils due to flooding greatly limit nitrification activity (4). The NO 3…”

Molecular basis of the nitrogen response in plants

“…In the field, nitrogen sources are not evenly distributed due to localized fertilizer application and the spatially inhomogeneous distribution of soil microorganisms with nitrification or denitrification activity (Hodge 2004;Hinsinger et al 2005;Gallardo et al 2006;Li et al 2016). To contend with the uneven soil distribution of nitrogen, plants sense nitrogen sources and subsequently change their root architecture to effectively exploit available nitrogen sources.…”

“…When both nitrate and ammonium are supplied in hydroponic cultures, plants favor ammonium uptake over nitrate (Sasakawa and Yamamoto 1978;Taylor and Bloom 1998;Gazzarrini et al 1999). In upland conditions (i.e., non-flooded conditions), ammonium availability is low due to competition with soil microorganisms and nitrification by aerobic microorganisms (Li et al 2016). Nitrate is the main nitrogen source for most plant species growing in upland conditions, where the nitrate:ammonium ratio reaches several dozen-fold (Miller et al 2007).…”

“…By contrast, the nitrate content is low in paddy fields due to its low electric potential and relative inactivity of nitrifying bacteria under hypoxic conditions. Ammonium is the predominant nitrogen source for plants growing in flooded conditions, where the ammonium:nitrate ratio in paddy fields can reach up to more than 10-fold (Li et al 2016). Although both nitrogen sources can be utilized by plants, the ratio of ammonium to nitrate that maximizes the biomass production differs among plant species (Falkengren-Grerup 1995).…”