Objective Leader - Dr Cecile de Klein (AgResearch)


This objective investigates ways to manipulate the N inputs into the pastoral system though plant breeding/selection and plant management options. The plant breeding/selection component focuses on exploring the hypothesis that novel pasture grasses can produce more DM per unit N supplied than existing grass species to produce high yielding plants with low N content. This research combines molecular biology, plant physiology and ecology to alter the fundamental biology of forage plants&#39; strategies for utilising N that has already been captured by the plants and convert it into DM. 


High yielding plants with low N content will not only reduce the total N input to the system, but also alter the N concentration of animal urine deposited to pasture. Over 80% of the national N2O emissions are derived from animal urine and the effect of N concentration on N2O emissions can have a major impact on total emissions. This objective will establish the relationship between N concentration in the urine and N2O emissions, to fully assess the effectiveness of this mitigation option (and other options that will impact on the N concentration of animal urine) on reducing the agricultural N2O emissions.


The plant management component of this objective focuses on understanding and manipulating N2O emitted by leaves. Previous work has shown that plants can emit N2O in three ways: 1. N2O produced by soil micro-organisms is transported to the atmosphere through the plant. 2. N2O is produced by microorganisms (ammonia oxidizing bacteria (AOB)) on plant leaves 3. N2O is produced by the plant during photo-assimilation of nitrogen. The work in this objective will quantify the contributions from each of these pathways and screen plants for variation in their potential to emit N2O. 


Key research questions:



	
	
	1. What are key gene targets that regulate plant growth?
	
	
	
	
	2. Can exogenous growth stimulants be developed that promote plant growth under N limitation?
	
	
	
	
	3. What is the relationship between urine N concentration and the N2O emission factor?
	
	
	
	
	4. What is the relative contribution of plant canopy N2O emissions to total pasture emissions?
	
	
	
	
	5. What is the relative contribution of the three different pathways to N2O emissions from plant canopy?
	
	



Significant new knowledge:



	
	
	Putative key molecular factors limiting plant growth under N limitation; 
	
	
	
	
	The potential for developing novel pasture species that can produce more DM per unit of N supplied; 
	
	
	
	
	Assessment of the impact of developing these species on N intake, milk yield, N excretion rates and N2O emissions; 
	
	
	
	
	Assessment of the effectiveness of N2O mitigation strategies that alter the urine N concentration and N excretion rates;
	
	
	
	
	Assessment of the relative importance of N2O emissions through plant canopy in relation to total pasture N2O emissions.
	
	



2.1 - Progress in 2009/2010


Work has been completed in three milestones, each pertaining to a separate area of work: 1) exploring the hypothesis that novel pasture grasses can produce more DM per unit N supplied than existing grass species; 2) assessing the effect of urine N concentration on the N2O emission factor, and 3) quantifying the importance of N2O emissions from the plant canopy.



	
	
	An early career scientist has been successfully recruited and a project plan and key experiments have been designed and presented at a N2O NZAGRC meeting on 24 June 2010. Based on extensive literature review and experimental evidence we have identified as key gene targets linking plant growth to nutrient and other environmental conditions gene products which are related to the reception, synthesis, and signalling of gibberellins, which are key regulators of plant growth
	
	
	
	
	Two field experiments have been established to determine the effect of urine N concentration on the N2O emission factors. The rates of N applied equate to 200, 400, 600, 800, 1000 and 1200 kg N/ha, and thus span the urine N deposition rates common to pastoral animals.
	
	
	
	
	An experimental set-up to systematically quantify different pathways of N2O release from the plant canopy has been designed and presented at a N2O NZAGRC meeting on 24 June 2010. This set-up uses a hydroponic system with ammonium as an N source only to minimise any N2O emissions other than those from the plant canopy
	
	



2.1 - Progress in 2010/2011


Trials to confirm the critical importance of gibberellins in regulation of growth ryegrass were undertaken.  Our results clearly show that defoliation of ryegrass results in a shift from reserve carbohydrate (CHO) accumulation to CHO mobilisation and that this is regulated by endogenous levels of gibberellins.  This endogenous supply of gibberellins results in the growth of tissues, which because of defoliation, are resource deprived.  A second series of trials has been initiated to test the effect of an exogenous supply of gibberellins on plant growth when N is the resource limiting plant growth.  A peer-reviewed paper on the effects of defoliation on gibberelin synthesis has been drafted.


It is still an open question as to whether urinary N concentration itself influences the quantity of N2O emitted per unit of N deposited in urine patches.  This has important implications both for inventory and for potential mitigation practices.  Field studies undertaken this year suggest that N2O emission per unit of deposited N is relatively constant for low urine N concentrations between 2 and 8 g N/l, but tended to increase at high concentrations of 10 and 12 g N/l.  Typical urinary N concentrations in New Zealand are 8g N/l.  Initial indications arising from the first year of these studies are (1) that N2O mitigation strategies that reduce urine N concentration from ‘typical&#39; to ‘low&#39; but don&#39;t reduce the total amount of N excreted, might have limited impact on reducing N2O emissions and (2) that the constant figure currently used in national inventory calculations is appropriate.


A prototype experimental unit consisting root and shoot chamber was constructed to test pasture canopy N2O.  This experimental unit allows the team to measure and quantify N2O emissions from pasture plants and to assess the contribution of this loss pathway relative to N2O losses directly from soil.  A series of experiments have been conducted during 2010/11.  Testing of the experimental unit identified some issues relating to air tightness of the unit, and an improved design was developed with the AgResearch engineering team.  This improved unit is now being built and will be tested by 31 July 2011. 


DISCLAIMER:  The above reports have not been peer reviewed and report interim results only.  Therefore, they may be subject to change.