[en] Although ordinary perturbative Green functions in two dimensions are not defined for the non-linear O(N) sigma model, one can consider the model in the presence of a long-range external magnetic field. It is shown that this model exhibits in perturbation theory two quasi-conserved local currents for which the breaking terms are proportional to the external soft source term. Under usual assumptions about the spectrum of the non-linear O(N) sigma model, the second quasi-conservation law can be taken in the limit of vanishing external field; one then recovers the conservation of the third moment of energy-momentum vector. (orig.)