~esigning a Long-term Nitrogen Balance Experiment

The answers to many agronomic questions can only be obtained from long-term experiments in which the evolution of responses to experimental treatments can be observed for several years. In this paper, we describe our experience in designing and running an experiment set up to study changes in the nitrogen balance associated with a continual rotation of summer and winter crops grown on a Tokomaru silt loam following pasture. We emphasize the importance of careful definition of the objectives of a long-term experiment, and we discuss the role of mechanistic models in such experiments. We point out that it is most necessary to review the progress of the experiment critically, and at regular intervals, lest the objectives slip out of focus and the experiment degenerates into an exercise in data collection. INTRODUCTION There is an important category of agronomic experiments where long-term research is necessary. This category embraces experiments. in which the effects of treatments are manifest only after periods of time running into years. The classical experiments of this kind are the continual cropping trials which have run in the United States for over ninety years (Russell, 1975; Richards, 1978), and in the United Kingdom for more than a century (Russell, 1961). On a more modest scale, examples of long-term agronomic experiments in New Zealand include the work of Sears: et al. (1965) on repeated cropping following pasture, the study of Douglas et al. (1972) on crop rotations involving maize, and the investigations of the long-term effects of cultivation techniques by Sims (1978). The common feature of all these experiments is that they were concerned with the evolution of responses to treatments over relatively long periods of time. In contrast, the great majority of agronomic experiments are concerned with the comparison of treatment effects over a relatively short time period, commonly within a season or a year. Although many of these experiments may be repeated in several years, this amounts to replication of the experiment in time rather than a study of changing responses to experimental treatments. In this paper, we shall take the latter feature, the evolution of response over several years, as the distinguishing characteristic of the long-term experiments we wish to discuss. We note that relatively few of the crop agronomy experiments described in the New Zealand literature in recent years meet our criterion, whilst a number of grassland experiments, particularly those on the farmlet scale, do. However, we shall restrict our discussion to long-term crop agronomy experiments. Our objectives in this paper are to outline the factors we considered in designing such an experiment and to discuss some of the difficulties we have fac~d in running it, in the hope that our experience will benefit others. 29 A LONG-TERM STUDY OF THE NITROGEN BALANCE A long-term nitrogen balance study was initiated at Palmerston North in 1976. The immediate objective of this experiment is to quantify components of the nitrogen balance of a continual double cropping system. A rotation of summer and winter crops (originally maize/oats, now barley/oats) has been established on a paddock which had previously been under pasture for five years. The soil is a Tokomaru silt loam, an Aeric Fragiaqualf. Three treatments are being used to create long-term changes in nitrogen status: application of nitrogen fertilizer to the summer crop with return of above-ground crop residues, application of nitrogen with minimal return of residues, and no nitrogen and minimal return of residues. The major emphasis at present lies with the second and third treatments, where the components of the nitrogen balance are being monitored at freq uent_imervals. Defining the problem and setting objectives The motivation for the study stemmed initially from the need for increased understanding of the nitrogen requirements of forage cropping systems involving both winter and summer crops. In most New Zealand farming systems, forage crops are used in short-term rotations between periods in pasture, with little consequent need for additional nitrogen. With the prospect of larger.,cale, longer-term implementation of forage cropping (Stephen and McDonald, 1978; Taylor and Hughes, 1978), and with evidence of responses to nitrogen appearing after a few years, even on soils with relatively high organic matter status (Sears et al., 19 65; Douglas et al., 1972), there is an obvious need to define more precisely the nitrogen fertilizer requirements of· double cropping systems. We started, therefore, with the question how do the nitrogen fertilizer requirements alter during a· continual seque·nce ot ·crops? To answer this question in a practical fashion, Proceedings Agronomy Society of New Zealand 9; 1979 several pieces of information are required. First, we need to know what levels of available N are necessary so that lack of N does not limit crop growth. If we can define this requirement, we must then determine how much N can be supplied from the soil. The difference, suitably weighted to account for incomplete utilization, defines the fertilizer requirement (Parr, 1973; Stanford, 1973) thus: Nf = (Ny Nnm Nu )/E (l) In (1), Nf is theN fertilizer requirement, Ny is the requirement for optimum crop growth, Nnm is net mineralization of N from soil organic matter, Nt 1 is the mineral N (N0 3 • and NH4 +) in the root zone at planting, and E is the fraction ot Nf recovered by the crop, an efficiency factor. Since there are four quantities in (l) which must be known before Nf can be calculated, a worthwhile experimental objective would be to estimate their values. Unfortunately, difficulties immediately become apparent. Although Ny, and possibly E, might be fixed quantities for given crops and soils, Nnm and Nt 1 are not. Both the residual N in the profile at planting, and the net amount of mineralization are likely to change with the number of crops after pasture, with season, and with soil type. Furthermore, a measure of Nti at the beginning of the season and an estimate of Nnm would only be of use if all mineral N in the profile were available to the crop. This is not likely, particularly for a winter crop where leaching and denitrification can occur. These considerations led us to a closer examination of what we meant by "mineral N available to a crop". Plants draw their nitrogen from a pool of soluble N in the soil. This pool is subjected to the various losses, gains and exchange processes which are summarized in figure l. Since the law of conservation of matter must hold for any species of N, we know that (inputs)(outputs)= change in storage (2) for any defined pool. Let us consider the mass balance for the mineral N over the whole of a growing season within the root zone of a crop. We write (Nf + Nm)(Nv +Ne+ Nd +NI + Ni) = Nt2 -Nti(3) where the terms on the left-hand side are defined in figure 1, and where Nu and Nt2 are, respectively, the amounts of mineral N in the root zone at the beginning and at the end of the growing season. (In writing (3 ), we have assumed that the net exchange of NH4 + and loss of mineral N by runoff are zero). Matching (3), we can write a mass balance for the organic nitrogen in the root zone: NiNm = Not2Noti (4) Here, N0 t 1 is the soil organic N present at the beginning of the season and Not2 is that present at the end. Equations (3) and ( 4) provide a summary of the changes which can occur during a growing season. We expected that the relative importance of the terms in (3) and (4) would vary from summer to winter, and that we would observe trends occuring in the terms as the experiment progressed. We found relatively little information had been published on these changes in New Zealand and knew of no experiment in which a complete N balance had been attempted for a crop. We decided, therefore, that a primary objective of our experiment should be to quantify the N balance of the double cropping system and to measure the 30 Figure 1: Schematic representation of the possible gains, losses and exchange processes affecting the pool of soluble nitrogen (N0 3and NH 4+) in the soil.